A wind mill has order 3 rotational symmetry. Did you notice that after the third rotation, it looks exactly as it looked at the start? We got total of three instances where the wind mill looked exactly like as the start.
The key-point is the final position of any object, after 360° rotation will always look like the initial object. So, every object will have rotational symmetry of order 1.

Let us move on to the question now.
                                         

In 1, for 360° rotation, we have two instances where it looks like the starting position. That is after 180° and after 360°. So, the order is 2 which will be the value of ‘A’.

But fig. 2 has order 1 because it only repeats itself after 360° rotation. So, B = 1.

That means A + B = 2 + 1 = 3