The correct answer is: A. into
The preposition “into” is used to indicate movement into a place or position. In this case, the man is moving into his armchair.
The preposition “over” is used to indicate movement across or above something. In this case, the man is not moving across or above his armchair, so “over” is not the correct answer.
The preposition “to” is used to indicate movement towards a place or position. In this case, the man is not moving towards his armchair, so “to” is not the correct answer.
The preposition “at” is used to indicate a position or location. In this case, the man is not in a position or location, he is moving into a position or location, so “at” is not the correct answer.