Explanation:

Virtual memory is a technique used by the operating system to provide an illusion to a process that it has access to a large, contiguous space of memory, which may not actually be physically present in the computer. The size of virtual memory depends on the size of the address bus.

Address Bus:
The address bus is a set of wires that carries memory addresses from the CPU to the memory. The number of wires in the address bus determines the maximum addressable memory space. For example, a 32-bit address bus can address up to 2^32 memory locations, which is approximately 4 GB.

Main Memory:
Main memory, also known as RAM (Random Access Memory), is the physical memory present in the computer. The size of main memory determines the maximum amount of data that can be stored in memory at any given time.

Data Bus:
The data bus is a set of wires that carries data between the CPU and the memory. The size of the data bus determines the maximum amount of data that can be transferred between the CPU and memory in a single clock cycle.

None of these:
None of the given options determines the size of virtual memory.

Conclusion:
In conclusion, the size of virtual memory depends on the size of the address bus, as it determines the maximum addressable memory space. The size of main memory and data bus are important factors, but they do not determine the size of virtual memory.