The correct answer is B. He proposed going for a swim as it was quite fine.
The speaker is suggesting that the listener go for a swim. He uses the word “propose” which means to suggest something as a plan or course of action. He also says that the weather is “quite fine” which makes it a good time to go for a swim.
The other options are incorrect because they do not accurately reflect the speaker’s meaning. Option A is incorrect because the speaker is not asking a question, he is making a suggestion. Option C is incorrect because the speaker is not simply suggesting going for a swim, he is proposing it. Option D is incorrect because the speaker is not giving advice, he is making a suggestion.