The answer is 4. 5 cannot be the answer, as it is not a factor of 16 or 32. The other 3 options are all possible. However, 16 cannot be the answer, as it was one of the factors already shown in the question. 32 cannot be the ans as 4 can also be a factor as they are all factors of 32. The largest factor is the number. So if 32 is the largest factor, aka the no., 4 will also be a factor. So 4 has to be in the qn, so the ans is 4, because it is also a factor of 16, which in this case, is the largest factor aka the number.

I’m sorry if u still don’t understand as I can only show working w secondary sch method which u hvnt learnt yet so u wont understand.