To solve this question, the user needs to know the full form of MCA and its maximum speed.

MCA stands for Micro Channel Architecture, which is a type of expansion bus used in computers. The maximum speed of MCA is measured in MHz (megahertz), which is a unit of frequency equal to one million hertz.

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

A. 4.77 MHz: This option is incorrect because 4.77 MHz is not the maximum speed of MCA. It is possible that some specific MCA devices may have operated at this frequency, but it is not the maximum speed.

B. 8 MHz: This option is incorrect because 8 MHz is not the maximum speed of MCA. While some early MCA devices may have run at this frequency, the maximum speed of MCA was higher than 8 MHz.

C. 10 MHz: This option is correct. The maximum speed of MCA is 10 MHz. This means that the MCA bus can transfer data at a rate of up to 10 million cycles per second.

D. 12 MHz: This option is incorrect because 12 MHz is not the maximum speed of MCA. The maximum speed of MCA is 10 MHz.

Therefore, the answer is: C. 10 MHz