Something is added to the triangle of A to make B. Add the same kind of thing to C.

In B an orange dot has been placed on every corner of the triangle: 3 corners, 3 dots. A square has 4 corners, so the answer has the square with a dot on each of its 4 corners: (c). (a) misses a corner, (b) puts the dots in the middles of the sides and (d) is a pentagon.