The critical damping of a spring mass-dashpot system

Question

A vibratory system consists of a mass 12.5 kg, a spring of stiffness 1000 N/m, and a dashpot with damping coefficient of 15 Ns/m

1. The value of critical damping of the system                                                                                

(a) 0.223 Ns/m                                                    (b) 17.88 Ns/m

(c) 71.4 Ns/m                                                      (d) 223.6Ns/m