Correct Answer - C
In the frame (inertial w.r.t. earth) of free end of spring, the initial velocity of block is `3 m//s` to left and the spring unstreched.
Applying conservation of energy between initial and maximum extension state.
`(1)/(2) mv^(2) = (1)/(2) kA^(2)` or `A = sqrt((m)/(k)) v= sqrt((4)/(10,000)) xx 3 = 6 cm`.