So the question is asking which pair of fractions in the box would have the same common denominator. Basically, look at the denominator of each fraction pair and see if both numbers can go into each common denominator you were given. I'll explain further.
Common denominator: 24) The pairs that correspond to this denominator is 3/4 , 2/6 because the denominators of both fractions (4 and 6) go into 24 evenly.
Common denominator: 15) The answer is 1/3 , 3/5 because 3 and 5 go into 15 evenly.
Common denominator: 9) The answer is 2/3 , 4/9 becuase 3 and 9 go into 9 evenly.