The correct answer is square 1.
First visible pattern: Between squares 3 and 4, as well as between squares 5 and 6 each of the upper letters rotates by 180 degrees from left to right; the lower letter is mirrored across a horizontal axis mirrored. This understanding is sufficient in order to solve the question, regardless of the unknown nature of transformation from one pair to the other.
Second visible pattern: in frames 4 and 5 – both letters maintain their orientation but switch places. Although only a single example for this alternation is offered (4 -> 5), applying it on 2 -> 3 will also reveal the correct answer.
Recommended work approach - Elimination: One may notice that all upper ‘F’s and lower ‘P’s are different from one another in their orientation. In order to preserve this pattern, answer choices 4 and 5 must be eliminated. If one considers the sequence as made out of two parts, three squares each, then both answer choices 2 and 3 must be eliminated. In each part, both letters maintain their orientation between the first and second step and then change them in the third step. Choices 2 and 3 are inconsistent with this logic.
Another point of view that you might find helpful: focusing on objects instead of locations may reveal that when an object maintains its location the orientation changes, and vice versa. These two changes occur alternately, meaning that the missing frame should differ from frame 1 in the objects’ orientation but not in location, and differ from frame 3 in location but not in orientation. This logic will also lead you to the correct answer- 1.