Can a processor without stack pointer registers, have a subroutine calls and interrupts?
The Control Data Cyber series had no stack pointer, but it had a Pascal compiler. Seymour Cray says your book is wrong. (The Subroutine Call instruction that this computer isn’t supposed to have would store the return address into the location after the instruction it branched to, forming an instruction that could be used to jump back to the caller). If your instructor doesn’t know who Seymour Cray is then they shouldn’t be teaching. 

And there were microprocessors that handled interrupts by switching to a second set of registers. So the answer would be 4.