we have 2 balls with the same mass
just by the concept of elastic collision the speed will be exchanged
so m2 will have 6 m/s to the right
if we go by the equation
by conservation of momentum
Given here is
m1=m2
u1=6 m/s
u2= -4 m/s (negative sign for the direction)
v1=?
v2=?
Putting these values in the equation above
Result: Option C is correct