To solve the question, the user needs to know about the Zero flag and the SUB instruction in assembly language.

The Zero flag is a status flag in the processor's flag register that indicates whether the result of an arithmetic or logical operation is zero. When a program performs a subtraction operation, the Zero flag is set if the result of the operation is zero, and it is cleared otherwise.

The SUB instruction is used for subtraction operations in assembly language. In the instruction SUB AX, AX, the value of register AX is subtracted from itself, resulting in a zero value.

Now, let's go through each option and explain why it is right or wrong:

A. Set: This option is correct. Since the result of the SUB AX, AX operation is zero, the Zero flag is set.

B. Cleared: This option is incorrect. The Zero flag is set because the result of the SUB AX, AX operation is zero.

C. Unknown: This option is incorrect. The Zero flag is set because the result of the SUB AX, AX operation is zero.

D. None of the above: This option is incorrect because option A is the correct answer.

Therefore, The Answer is: A. Set