[ INTERVIEW QUESTION ] Virtual Machine (VM) disk cannot be resized.

Impressive answer! Why a Virtual Machine (VM) disk cannot be resized and you failed to extend the disk? Your best answer... Most likely the VM got a snapshot. Because when a snapshot is taken the actual VMDK file will be locked and the changes will be written to a new delta file. Due to this reason, the actual VMDK file could not be resized. 

It is important to make a good impression on whoever is interviewing you. You can also mention... Also, according to my experience (1) if I edit a VM's settings and then select one of its virtual disks, the option to resize the disk will be grayed out. (2) When I try to extend the disk using vmkftools for resizing, I will get this error: Failed to extend the disk. Failed to lock the file. (3) When I extend the disk using vSphere client, the changes will not get reflected.