Concept:Angular speed for periodic vibration is calculated from frequency using ω=2πf.Explanation:One complete cycle equals 2π radians.The body vibrates at 50 cycles per second, so f=50 Hz.Thus, angular speed ω=2πf.Substitute f=50: ω=2π×50.This gives ω=100π rad/s.Answer:A. 100π rad s−1