i'm learning linux right now and my instructor told me that i can sit for any of the following certifications :
1/ Mandriva
2/ LPIC
3/ Suse
4/ Ubuntu

which one do you advise me to sit for ?
thank you in advance.

To me it would be a toss up between Suse and Ubuntu of the ones you listed. (The one you didn't list was RedHat Enterprise Linux and that would be the one I'd suggest if you had.)

RHEL and Suse are commercial distributions used in corporations that want the comfort of a vendor support. (In fact they are of course OSS - what you pay for is "subscriptions and support" not the OS.) The main reason for certification would be to use it as a credential for getting a job so focusing on a commercial distribution makes the most sense. RHEL has the lion's share of the commercial market.

Ubuntu is a distro of choice for many Linux enthusiasts. It too has paid commercial support available but most of its users aren't doing that. However, there are some folks using it in commercial settings without paying for support.

The idea is that if you're doing a technical interview Ubuntu might win you points but recruiters/HR and management often focus on experience with what they use so the commercial distros would likely mean more to those folks. Remember in a job search you have to get through recruiters and/or HR before you get to see the hiring manager or technical interviewers.

Like jlightner said, RHEL and SUSE are pretty much industry standard when choosing Linux for enterprise uses. RHEL has the most market share, but since it's not on your list, go with SUSE. If you can do a RHEL cert, it would probably be better, but if not, the SUSE cert is the way to go.

FYI: I tend to like SUSE better than RHEL. My Dad is an enterprise AIX consultant and his company also does Linux consulting. He and most the people that work for him (who are experts in their field) prefer SUSE to RHEL, however, their customers tend to choose RHEL since it is more widespread and well known. I'm no expert but perhaps we have a situation like Intel's P4 against AMD's athlon; P4's still sold well 'cause their marketing was much better and they were more well known. IMO

I'd also have strongly suggested RHCT/E, if that had been an option. It's by far the most widely recognized and respected Linux cert, IMO.

From the list you posted, I'd probably select LPIC.

It is true in the US that RH has the lion's share of the commercial market, but in Europe its much more even (but, even here, the RH Qual has more market prescence). But as the OP doesn't say where he is located, its unclear which geographical area is important to him.

Not even clear what the OP wants the qual for; if he intends to use it as a 'door opener' for a job interview, then out of the list given probably SuSE is the best choice; if its for 'home/general/someone else is paying for the training' purposes then doing training on a distro that he/she is likely to use is more likely to be best.

(Oh no, if this turns into a 'which is the best distro' thread, it'll all be my fault!)

Shrug...

"Certifications" bill($$) themselves as "the Golden Ticket," but I cannot help but look quite-skeptically at those claims. If your instructor is saying that you can now take "any one, pick one," then, by all means, 'pick one.'

When you're finished, presumably, you will "have one." (I will now suppress my urge to say, 'W00t! W00t!'...)

Enjoy "the brief warm glow of accomplishment," then, move on.

As you complete your training, you'll need to start positioning yourself in the job-market. This is, perhaps, not quite as difficult as it sounds: there are lots of "buyers" out there, and you only need to find one of them. Nevertheless, you should begin to seriously contemplate your angle. In other words, "within this great-big space called 'Linux,' what exactly would you like to do?"

The LPIC is for "generic" certification, meaning you can manage whatever distro you like.

From the FAQ on the LPI site:

To start with, I'd give LPI a shot and then choose RHEL or NOVELL Linux certs. The Ubuntu cert is not very challenging in my opinion (you need to do LPI 1 anyway before you can do the Ubuntu cert), its concentrates a lot on the desktop with very few questions about servers.

The only problem with that is that employers who require certification are typically requiring vendor specific because they use that vendor's product.

It is much the same in the UNIX world. I've been doing UNIX full time since 1991 but if I walked into an AIX shop with as little AIX (IBM's UNIX) experience as I have they'd pass on me for the next candidate that had more AIX experience. I once got a job simply because I'd worked for the previous 3 years on the Sun E10K. People with far more Solaris experience than I had at the time couldn't get an interview because they hadn't worked on that specific Sun model.