If two fractions have the same denominator, that means the total number of parts is the same. For example, you have two batches of cookies, which each have 8 cookies.
But the numerator tells you the number of parts you are dealing with. Going back to our cookie example, say you ate 3 cookies and your little brother only had 1. If you ate 3 cookies, you ate 3/8 of the cookies. Your little brother ate 1/8 of the cookies.
So, which one is the greatest fraction? In this case, who had more cookies? That would be you. So, the greater numerator means a greater fraction.
Answer: the fraction with the greater numerator