Answer : $13ms^{-1}\text{ in the direction to motion of the belt}$

Let us choose the positive direction to be the direction from P to Q.i.e., the direction of motion of the belt.Then,the velocity of the belt is

$v_B=+4ms^{-1}$

Speed of stone with respect to the belt is

$v_{SB}=+9ms^{-1}$

If $v_S$ is the speed of the stone with respect to a stationary observer,we have

$v_{SB}=v_S-v_B$

or $v_S=v_{SB}+v_B=+9+4=+13ms^{-1}$

The positive sign shows that the direction of velocity of stone is from P to Q,i.e in the direction of motion of the belt.Hence the correct choice is $13ms^{-1}\text{ in the direction to motion of the belt}$