You only have one drive, it is divided into two PARTITIONS.
While there is software that can merge partitions there is a danger it can corrupt your drive and you will lose everything.
You can use a D drive just as easily as a C drive.
Digital pictures, mp3 files, videos etc can all be stored on a D drive, you just need to specify D when saving the file.
In fact it is probably better to keep Windows and programs on C and personal files on D. That way if Windows crashes and you need to reinstal Windows your D drive with all your personal files will not be lost.