Which figure must come next in the series?

We can see that the square frame, along with the four figures in the corners is getting rotated by 90 degrees in the clockwise direction in each subsequent step. Therefore, only options A and C are logical continuations.
In the first figure, triangle ( 3 sided figure) is included in the centre.
In the second figure, square (4 sided figure) is included in the centre.
Therefore, in the answer, a hexagon (6 sided figure) must be included in the centre. Therefore, option C is the right answer.