There are 16 registers, and ALL can be USED as a base EXCEPT for register 0, so the answer is AT LEAST 15. AT LEAST is SPECIFIED here because in any section of code, you can "re-use" a previous base register once you are no longer within the original address range ("scope") of that PARTICULAR base register. By re-using the registers, you can have base registers that will cover ALL of the memory in the machine - but not all at once - you have to "bite off" 15 base reg-at-a-time chunks of memory (all addressable memory does not have to be contiguous - it can be scattered around memory in 4K pieces).

There are 16 registers, and ALL can be used as a base EXCEPT for register 0, so the answer is AT LEAST 15. AT LEAST is specified here because in any section of code, you can "re-use" a previous base register once you are no longer within the original address range ("scope") of that particular base register. By re-using the registers, you can have base registers that will cover ALL of the memory in the machine - but not all at once - you have to "bite off" 15 base reg-at-a-time chunks of memory (all addressable memory does not have to be contiguous - it can be scattered around memory in 4K pieces).