You can only add the unallocated space to the partition showing next to it. I is possible to eventually add it to the C: drive if you MOVE the other partitions along the drive until the unallocated space is adjacent to C:. You will then find that the EXTEND function won't be 'greyed out' and you will be able to use it to add unallocated space to C". Be certain that you have back ups of all your data as the process is not without some danger of corruption of the partitions, particularly if you are getting short of free space.
Surface Pro (2017) vs Surface Pro 4