Explanation

This uses the arc-length formula s = rθ with θ in radians. The pendulum end sweeps an arc of 14.3 cm through 9°, and 9° = 9π/180 = π/20 radians. Hence r = s/θ = 14.3/(π/20) = 14.3 × 20/π = 286/π. With π = 22/7, r = 286 × 7/22 = 2002/22 = 91 cm. Hence option (b). The distractors 88, 95, 98 cm result from rounding θ to 0.15 rad or from using the diameter form of the formula instead of the radius.