After opening Disk Manager, right-click on Disk Management and choose Rescan Disks (this should be unnecessary unless you hot-swapped the drives, this should happen during a reboot, still, may as well do it again).
After you have done that, you should see your Partition along with some Unallocated space. Just right-click on your partition and choose to expand it. Take all the default options and it should expand to fill the entire drive.
If for whatever reason you have unallocated space before the partition or on both sides of the partition, then you will need to use a tool to move the partition in order to fully use the space. AOMEI Partition Assistant is a good free tool for this purpose. Be warned, moving a partition takes a LONG time.
Another thing that could have happened. The hidden System partition may have been expanded when you cloned the drive. If that happened, that is not needed. You should be able to use AOMEI Partition Assistant to shrink it back to 100 MB, then expand your C drive to fill the entire drive.
Warning, be sure to backup your data before doing this. Any number of things could go wrong that could result in data loss or OS being unbootable.