The speed limit in a school zone is:

Answer: B — 20 mph

The speed limit in a school zone is generally 20 mph for the safety of children and students entering or leaving the school premises. Option A, 15 mph, is too low and may cause congestion and delays for drivers. Option C and D, 25 mph and 35 mph respectively, are both too high and can pose a safety risk to the students and those in the surrounding area. Therefore, B is the most appropriate and safe speed limit for a school zone.