How to use dd command to create a file test.img of size 50MB, containing all the zeros, which can be done by copying the device file /dev/zeros?

Try
dd if=/dev/zero ibs=1k obs=1k count=51200 of=test.img

can u explain about it?

Well.... I never use dd before (or maybe yes but I don't remember), I read the man page only. This works on my system, the options are:
if -> the input file
of -> the output file
ibs -> the input block size in bytes (1k = 1024 bytes = 1 kilobytes)
obs -> the output block size in bytes
count -> the number of blocks you want to copy from if to of (51200 blocks * 1024 byte/blocks = 52428800 bytes = 50MB)

you can use also:
dd if=/dev/zero of=test.img ibs=1M obs=1M count=50
i'm working on solaris at the moment, it haven't the M multiplicative suffix. I think this is a GNU extension.