Which file type must be selected to copy a single hard disk from one datastore to another?

To copy a single hard disk from one datastore to another in a VMware environment, the correct file type to select is the Virtual Machine Disk (VMDK) file. VMDK files are used to store the virtual hard disk of a virtual machine, containing the operating system, applications, and data of that virtual machine. They are critical to the functionality of virtual machines because they represent the virtual hard drive where the virtual machine reads from and writes to.

When performing operations such as cloning or migrating a virtual machine's disk, the VMDK is the file that must be manipulated, copied, or moved to ensure that the virtual machine can be successfully launched in its new location with all its data intact.

The other file types mentioned serve different purposes. VMSS files are related to suspended virtual machines, containing snapshot states but not the actual data disk. VSWP files are swap files created when virtual memory is used, which are not needed for the disk transfer itself. VMX files are configuration files that contain settings and configurations for virtual machines but do not represent the disk data. Thus, when moving a hard disk, it is the VMDK that is essential for maintaining the integrity of the virtual machine's disk storage.