Which of the following answer figure completes the series of the question figure?

In the first and the third box, the triangle is in the same place without any rotation. Similalry, in the second and fourth box, the triangle will be in the same place without any rotation. By this option (d) can be eliminated. Because triangle was rotated there.

In the first and the third box, the circle is in the same place. Only the filling part of the circle is changed diagonally from first to third box. Similarly, in the second and fourth box, the circle will be in the same place. Only the filling part of the circle is changed diagonally. By this option (a) and (b) can be eliminated.

Hence option (c) is the correct answer.