Which side lengths form a right triangle?
(look in the picture for the answers)

Pythagoras' Theorem only works with right angle triangles. 

A  makes a right angles triangle because:
   √3² + 4²  = 5

However B doesn't because:
√7² + 8² = 10.631  (which is not 9)

Nor does C because:
√3² + 9² = √90   (which is not √95)

So the answer is A