This appears to be Windows Vista or Windows 7, but you don't say.
From the image it looks like someone has given you the computer's hidden Recovery partition. I don't know how they did that. The hidden recovery partition is what is used when we want to reinstall the OS, (Operating System), and set it back to factory conditions; that is, 'day 1'.
I doubt you can do that now because it looks like that hidden partition is no longer hidden, and has been used to install the OS. That limits you immediately to how much you can install or save on that partition. At 14.6 GB much of the partition is going to be used up by Windows and just a few necessary applications.
If you have any videos, music, pictures, documents, or other personal files on that partition, move them to the other, larger, partition. That will give you some grace, but not much.
Any software you want to install, check the installation procedure carefully. Often you will be given a choice where to install it. Use the Browse button to select the D Drive. But beware, most software installations will still place files and other application components onto the main C drive. There is nothing you can do about that.
But even then, some software will not give you the choice of where to install. They will install on the C drive, and only on the C drive.
That is why you are going to struggle. That and all the other temporary files and internet files that always build up on a system through normal use.
If this were me, I would start again. Delete the partitions altogether, reformat, and reload the OS onto just the one hard disk, no partitions.
May be the person who did this to you in the first place can do that, and make this a workable system.