If you want to make it all one partition.
1) back up everting on D drive that you want to keep.
2) Go to disk manager
.. Rt Click on My Computer, select manage.
.. On left pane, select Disk management.
3) right click on D drive. and select delete volume.
after you have deleted D drive volume.
4) Rt click on C drive and select expand, expand to full size of drive.