Final answer:
The hard drive speed is measured in Revolutions Per Minute (RPM), which indicates how many times the drive spins per minute.
Step-by-step explanation:
The hard drive speed refers to how fast the hard drive can spin its disks to read and write data. This speed is measured in Revolutions Per Minute (RPM), which is a measure of how many times the drive can spin in one minute. Therefore, the correct unit of measurement to indicate hard drive speed is Revolutions Per Minute (RPM). Options such as Gigabytes (GB) and Terabytes (TB) refer to storage capacity, while Megahertz (MHz) is used for measuring the frequency of electronic devices, including the CPU clock speed.