If you have a single physical drive split into multiple partitions.
As an example from C: to E: and each partition is of a set size.
And there is no physical free space left on the single drive split into partitions.
You must first shrink the size of a partition.
For example say you had a D: partition on the single drive and it was 25Gb in size and there was free space left on it of 10Gb.
You would shrink the D: drive by either 10Gb or less say 9Gb.
Once you have done this.
You can then click on the C: drive and expand or add the 9Gb or 10Gb of space to the C: drive.
You must if the single drive is partitioned multiple times say four giving you a Drive C: D: E: and F:
shrink one of the other partitions down in size so the free space is not associated, allocated to another Drive volume label, or drive letter.
Then you can increase the Drive size, or capacity of the C: partition.
By adding or setting the drive size plus the 10Gb to it you have freed up, from another partition on the single drive.
Each Partition can be shrunk if there is free space on it.
Just total up how much you shrank each partition as an example.
Then as said add the total to the C: drive, partition.