To determine which instruction(s) can be performed only in the CX register, we need to understand the purpose and limitations of the CX register.

The CX register is a 16-bit general-purpose register in the x86 architecture. It is commonly used as a loop counter in assembly language programming. The CX register can be modified by instructions such as LOOP and is used for controlling the number of iterations in a loop.

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

A. LOOP: This option is correct. The LOOP instruction is used to create a loop in assembly language programming. It decrements the CX register by 1 and jumps to the specified label as long as the CX register is not zero. The LOOP instruction relies on the CX register and is commonly used for iterative operations.

B. MUL, DIV, CBW: This option is incorrect. The MUL (multiply), DIV (divide), and CBW (convert byte to word) instructions do not specifically depend on the CX register. These instructions can work with other registers, such as AX, DX, and AL, based on the requirements of the specific instruction. They are not exclusive to the CX register.

C. ADD, SUB: This option is incorrect. The ADD (addition) and SUB (subtraction) instructions do not depend on any specific register. They can be used with various registers, including the CX register, based on the instructions' requirements. They are not limited to the CX register.

D. none of the above: This option is incorrect. As explained above, the LOOP instruction can only be performed in the CX register. Therefore, option A is the correct answer.

The answer is: A. LOOP