Explanation
Count multiples of 4, subtract century years not divisible by 400, and retain centuries divisible by 400. The verified count in [3200, 3280] is 21. Option B (22) is rejected because it does not equal the computed result; it represents a counting, direction, leap-year, interval, or hand-motion error. Option C (20) is rejected because it does not equal the computed result; it represents a counting, direction, leap-year, interval, or hand-motion error. Option D (23) is rejected because it does not equal the computed result; it represents a counting, direction, leap-year, interval, or hand-motion error.