Answer
Please see the work below.
Work Step by Step
We know that the lifeguard who follows the path $ACB$ runs more distance on sand as compared to the other; hence, that lifeguard takes less time to travel more distance. The distance traveled in water by lifeguard to path $ACB$ is less than the other, so we know that it takes less time to reach point B since the speed of both lifeguards is the same.
