The correct option is
B Q-P-R-S
The first statement in the sequence should be the one that introduces a new topic. It must not be a continuation or extension of a topic.
Q is first in the sequence: It initiates a topic on 'he', the subject. All other phrases continue the discussion on the subject and thus, Q comes first. It says 'he stopped'.
Q is followed by P: P further adds that he stopped in the middle.
P is followed by R: R continues by telling us what he stopped in the middle of what. It says that he stopped in the middle of his speech.
R is followed by S: S tells us why he stopped. It says that he stopped in the middle of his speech to take a glass of water.
Finally, the correct line is 'He stopped in the middle of his speech to take a glass of water'.
Thus, the correct sequence is Q P R S. Hence, C is the correct option.