If the and a leg of one triangle are congruent to the hypotenuse and a of another triangle, then the triangles are congruent by the hl theorem.

Your answer is A.  Hypotenuse...Leg

This makes the entire sentence read as the following:

If the hypotenuse and a leg of one triangle are congruent to the hypotenuse and a leg of another triangle, then the triangles are congruent by the HL Theorem.

An easy way to remember this theorem and what it stands for is that the letters are HL--- the H stands for Hypotenuse and the L stands for Leg.

Hope this helps! :)