The correct option is B √mKv0
According to law of conservation of energy
Ei=Ef
Ui+Ki=Uf+Kf
[Sum of potential energy stored in spring and kinetic energy of block will be constant]
[Ui=0, as spring is unstretched]
0+12mv20=12Kx2
[for maximum compression Kf=0]
x=√mKv0
Maximum compression in spring, x=√mKv0