Merging two partition drives

There is no software that will merge the two volumes while keeping all of the data intact.

There is however, partitioning software which will for example, expand the C: drive and simultaneously shrink the D: drive by up to the amount of empty capacity on D: drive. See Paragon or Acronis for said software, for example.

If you still want to merge the volumes, you will have to
a) back up the D: volume (since playing with partitions, you "should" back up all volumes for safety)
b) delete the D: volume either with Disk Manager or with the partitioning software
c) use partitioning software to expand the C: volume to take up all of the capacity formerly held by D:
d) restore the data formerly on the D: volume to the newly expanded C: volume.

Note: if any applications were installed on D: or referred to data on D:, may need to be re-installed to refer properly to their new home on C: volume; shares formerly on D: will have to be re-created; and other references to D: volume will have to be fixed.

Your question has already been answered here:
https://www.techrepublic.com.com/5208-6230-0.html?forumID=101&threadID=339255&tag=content;leftCol