Answer:
5
Explanation:
4 and 9 are composite numbers, so that automatically rules them out. The last two Prime numbers are 7 & 5, right?
180/7 = 25.71 - Since this ends up being a decimal, it cannot be a factor.
180/5 = 36
5 is both a prime number, and it factors evenly: this makes 5 the correct answer.
Hope I could help!