how can the option which os to boot don't display in mint?
 

i've tried creating an option in grub.d/ and update-grub but after booting the options don't display and instead it boots mint directly.

how can i make it appear when booting so i can have the option to boot my second os?

I think you would need to post a little more information to get realistic help. What option are you referring to, how did you try to create it and what exactly did you do? What is your second OS? How many OSs do you have? What partitions do you have?

I'm not really familiar with Grub2 but this tutorial might give you the information you need:

http://www.dedoimedo.com/computers/grub-2.html

Are you referring to other operating system(s) or the Mint options?

You can edit the file /etc/default/grub, and comment out with a hash the line that says
#GRUB_HIDDEN_TIMEOUT=0
then make the timeout a reasonable amount
GRUB_TIMEOUT=10

dual boot. i'vecreated 11_secondOS.sh in grub.d/
chmod 777 it and run update-grub
successfully
but the option don't prompt upon booting..

the question is just how can i make it to prompt the options which to boot whether i'd like --Memory test upon booting.

it is actually commented out by default. but still not working

here is the /etc/default/grub
i've run update-grub after editing but still not working

There's a useful guide on tweaking Grub 2 here
I'm not sure that I haven't misunderstood you.
If what you posted above is what you're using, I'm not surprised that the menu doesn't show. Here's mine:

GRUB_DEFAULT=0
#GRUB_HIDDEN_TIMEOUT=0
GRUB_HIDDEN_TIMEOUT_QUIET=true
GRUB_TIMEOUT=10
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_CMDLINE_LINUX="setkmap=fr splash vga=773"

As you see, there is a # in front of GRUB_HIDDEN_TIMEOUT=0
The menu comes up when I boot, and I can choose which OS to boot into. If I don't make up my mind in 10 seconds, it boots into the default OS - the first one on the list, in my case.
Is this what you wanted?

You talk about "options". If you're referring to kernel options, they are specified in the last line of what I show above: setkmap=fr because I have an azerty keyboard, splash for the splash screen, and vga=773 for the screen mode. (This last is deprecated - should be vga=normal, I think - and I get scolded for it every time I boot :tisk: ).

If it's memtest86 that you want, make sure that the file /etc/grub.d/20_memtest86+ exists and is executable.
Hope this helps.