The correct answer is option 'A', a young boy threw Douglas into the swimming pool.

Explanation:
The passage suggests that Douglas was thrown into the swimming pool by a young boy. Let's analyze the options to understand why option 'A' is the correct answer.

a) A young boy: The passage implies that someone threw Douglas into the swimming pool. Since the passage does not provide any information about the identity of the person, it is reasonable to assume that a young boy was responsible for this action.

b) Instructor: The passage does not mention anything about an instructor throwing Douglas into the swimming pool. Therefore, it is not the correct answer.

c) Mother: The passage does not mention anything about Douglas's mother throwing him into the swimming pool. Therefore, it is not the correct answer.

d) Watchman: The passage does not mention anything about a watchman throwing Douglas into the swimming pool. Therefore, it is not the correct answer.

Based on the information provided in the passage, option 'A' is the most logical answer. However, it is important to note that the passage does not provide explicit details about the incident, so there may be some ambiguity in the interpretation of the answer.