To answer this question, the user needs to have knowledge of the 8086 processor and its limitations.

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

A. It has only 14 registers: This option is incorrect. The number of registers in the 8086 processor does not determine the maximum memory space addressable. The registers in the 8086 are used for various purposes, but they do not directly limit the addressable memory space.

B. The register size was 16 bits: This option is incorrect. The size of the registers in the 8086 processor does not directly determine the maximum memory space addressable. The register size affects the size of the data that can be stored and manipulated within the registers, but it does not limit the overall addressable memory space.

C. The address bus width was 20 bits: This option is correct. The address bus width of the 8086 processor is 20 bits, which means it can address up to 2^20 (1,048,576) memory locations. Each memory location can hold a byte of data. Therefore, the 8086 processor can address a maximum of 1,048,576 bytes or 1 megabyte of memory.

D. none of the above: This option is incorrect. Option C, which states that the address bus width was 20 bits, is the correct answer.

The Answer is: C