The fourth choice goes in the first box.
The second choices goes in the second box.
The fifth choice (the one on the bottom row) goes in the last box.
The fourth choice is 2/6. When reduced it equals 1/3.
The second choice is 3/6. When reduced it equals 1/2.
So when you add them, 2/6 + 3/6, you get 5/6.