Final answer:
The scenario where a computer's HDD LED is lit but the mouse doesn't respond and the computer is unresponsive typically means that the computer has locked up.
Step-by-step explanation:
The scenario described indicates that the computer is experiencing an issue where it has become unresponsive. This can be interpreted by the HDD LED being lit, which typically signifies that the hard drive is active, and the fact that the mouse does not respond. Given the choices, the most accurate description is that the computer has locked up (A). This would suggest that the computer has encountered a system error or software issue that has caused it to become unresponsive to user input. The other options, such as Windows Update running, being in Safe Mode, or the reset button being stuck, would not typically cause the mouse to be unresponsive and are therefore less likely to be the correct answer.