Question: 
I’m trying to extend ?the D: partition using unallocated 30.91 GB space at its left side.
Another thread states it can’t be done using Disk Management’s UI and it suggests using 3rd-party tools.
However, how can we do this natively (e.g. diskpart) without installing/downloading additional software?
Answer: Yes that is possible. Copy all the data from your D drive to a different disk/partition, delete the volume of D and then re-create it. Then move your data back on the new D drive.
That is the only way without 3rd party tools.