Which set of side lengths defines a right triangle?
a.
6, 8, 11
b.
5, 12, 13
c.
7, 9, 13
d.
6, 9, 11

In a right triangle, the square of the largest side is equal to the sum of the squares of the smaller sides, so if you plug the sides in to that equation, you should get zero if those sides form a right triangle.
B is the only option where this is true, so I believe set B defines a right triangle.