You can move files from D to C then delete D in Disk Management then Extend C into the space, but the benefit of having a separate data partition is it keeps your Windows Backup Image smaller without all of the storage included on it. Having a Windows 7 backup image and data files stored externally is the safest method and modern way to reinstall Windows 7 if it ever becomes irreparable or the HD fails. Backup Complete Computer - Create an Image Backup
To delete D and extend C: Partition or Volume - Delete Partition or Volume - Extend
To instead resize D to extend C into the space, Download free Partition Wizard
bootable CD, write ISO file to CD using Windows Image Burner, boot CD.
Rightclick on D to Resize, slide left grey border to right to make as much as space as you want to expand C into, click OK.
Now rightclick on C>Resize, slide right grey border to the right to take up the space you made from D, Click OK, Apply all steps.