At highest point velocity of particle is vcosθ=200 cos 60=100 m/s
Let the mass of particle be 3m
It is given that two particles of equal mass and same magnitude of velocity goes opposite to each other one goes up while other goes down, thus they cancel each other momentum.refer figure below
Now the last fraction of mass will have momentum equal to the momentum of particle before explosion
∴ 3m(100)=m(v') according to law of conservation of energy
∴ v'=300 m/s along x axis
Answer: (b)