The correct answer is C. 30 minutes.
The time of concentration is the time it takes for water from the most remote point in a watershed to reach the outlet. In this case, it rains between 2 P.M. and 3 P.M., but the entire basin area just starts contributing water at 3 P.M. This means that it takes 30 minutes for water from the most remote point in the basin to reach the outlet.
Option A is incorrect because 15 minutes is not enough time for water from the most remote point in the basin to reach the outlet. Option B is incorrect because 20 minutes is not enough time for water from the most remote point in the basin to reach the outlet. Option D is incorrect because 60 minutes is too much time for water from the most remote point in the basin to reach the outlet.