Final answer:
To start the BIOS setup program, a key must be pressed during the POST phase of starting the computer.
Step-by-step explanation:
To start the BIOS setup program, a key must be pressed during the POST (Power-On Self-Test) phase of starting the computer. During this phase, the BIOS checks the hardware components of the computer and prepares them for the operating system. Typically, the key to access the BIOS setup program is displayed on the screen during the POST process, such as 'Press DEL to enter setup' or 'Press F2 for BIOS'.
The key to enter the BIOS setup program should be pressed During POST (Power-On Self-Test), which is the correct answer to this question. This is the time when the computer performs preliminary checks on the hardware and prepares to load the operating system. It is prior to the operating system boot, so options B, C, and D are incorrect.