Hey there, dexter11!
Would you please post a screenshot of what you see about the drive and the partitions in Disk Management (you can use imgur.com to upload the image)? In order to be able to extend a partition via Disk Management, the unallocated space has to be right next to that partition on the right side (if we're talking about the OS drive). Otherwise if it's a secondary drive you can do that even if the unallocated space is not next to the partition you want to extend, but this turns the disk into dynamic and you can't boot with a dynamic disk, so that's why in your case the option is greyed out.
I'd recommend that you backup any important data which you have on that drive (especially partition C

, just to be on the safe side. After that, go ahead and download a 3rd party partition management program and try to extend your C: drive this way.
I'd recommend that you wait until I see the screenshot before you do that, just to make sure that this is the case.
Hope that helps.
Boogieman_WD