To answer this question, we can use the given information to determine the positions of the swimmers. Let's go through each option to understand why it is correct or incorrect:
Option A) 1 - This option is incorrect because the question states that F is ahead of D, which means there is at least one swimmer (D) between A and F.
Option B) 2 - This option is incorrect because the question states that A is behind D and E, and F is ahead of D. Therefore, there are three swimmers (D, E, and F) between A and F.
Option C) 3 - This option is incorrect because the question states that A is behind D and E, and F is ahead of D. Therefore, there are three swimmers (D, E, and F) between A and F.
Option D) 4 - This option is correct because the question states that A is behind D and E, and F is ahead of D. Therefore, there are four swimmers (D, E, F, and one other swimmer) between A and F.
The correct answer is D. There are four swimmers that separate A and F.