Answer:
3
basically, 8 can go into 24 exactly but 15 and 5 can not go into 15 perfectly neither can 24. 24/3 is 8 and 15/3 is 5
You might say two could do the same thing which is true but we are talking about the GCF, they are asking for the greatest common factor so that's why the answer is 3 :D