A block of mass m is attached to one end of a mass less spring of spring constant k . The other end of spring is fixed to a wall. The block can move on a horizontal rough surface. The coefficient of friction between the block and the surface is u . Then the compression of the spring for which maximum extension of the spring becomes half of maximum compression is
a. k2mgμ
b. kmgμ
c. k4mgμ
d. None of these