Which figure does not always have congruent diagonals?

a) square
b) rhombus
c) rectangle
d) isosceles trapezoid

If you cut a diagonal line across a square, both sides look equal. A rhombus also looks equal and so does a rectangle. However when you look at trapezoid, it has shorter top and a larger bottom. Cut that in half diagonally and you can see that one half would be smaller than the other. 

The third box is a function