What is the greatest common factor of 8 and 15?

The GCF of 8 and 15 is 1

The prime factors of 8 are 2, 2 and 2

The prime factors of 15 are 3 and 5

They have no common factors - other than 1. The GCF is 1.
8 and 15 do not have any common factors therefore the answer must be one (1).
It is: 1