Welcome to Seven Forums shamima,
Hope you are finding everything you need, lots of tutorials here
and a nice introduction: Welcome to the Windows 7 Forums Tutorial Section
You can stop by this thread and say hello: Welcome to Seven Forums 
Oh, BTW, this thread is 8 months old and the original poster has been Banned for not following the forum rules, so that person will never see your reply. Forum Rules
Disk management won't allow partition C to be extended into this unallocated space, although partition D could be extended into the unallocated space as it is to the right of partition D.
A great free program Partition Wizard Home Edition
can be used to add the unallocated space into partition C.
Partition Wizard and many more free programs can be found at FREE Great Programs for Windows 7
Recommended partitions are for the first partition to be 60 - 80GB for the operating system, at least one more for your personal files and Data. This will make any future re-installation of your OS easier and will not affect your personal files and data.
Hope this information is useful, we all appreciate anyone who helps out