Here are two sums

A = 2/15 + 1/3

and

B = 6/7 - 1/3

Which of the two sums is closer in value to 1/2?

2/15 + 1/3 = 2/15 + 5/15 = 7/15

6/7 - 1/3 = 18/21 - 7/21 = 11/21

1/2 - 7/15 = 15/30 - 14/30 = 1 / 30

1/2 - 11/21 = 21/42 - 22/42 = - 1/42

1/42 is smaller than 1/30

so sum B is the answer