Explanation:
the 2 triangles are similar.
that means their corresponding angles are equal, and their side lengths (and heights and so on) have all the same constant scaling factor.
so, we see that the side with the length 6 must corresponds to the side with the length 3 (factor 1/2).
as then the side with the length 5 corresponds to the side with the length 2.5, and the side with the length 4 corresponds to the side with the length 2 (all have the same factor 1/2).
a is the angle between 6 and 5 (the 2 longest sides).
the angle between the 2 longest sides in the smaller triangle (3 and 2.5) is d.
so, a and d correspond.
b is the angle between 6 and 4 (the longest and the shortest side).
the angle between the corresponding sides 3 and 2 is c.
so, b and c correspond.
therefore, the 4th and last answer choice is correct.