Question 7
Rocket works on the principle of conservation of
(a) mass
(b) energy
(c) momentum
(d) velocity

Since there is no external force on the rocket. The force with which the rocket gives out the fuel, the fuel applies the same force to the rocket(action-reaction pair). Since there is no external force, we can apply conservation of momentum as internal forces(burning of fuel) does not change the momentum of the whole system(rocket + fuel).Hence option C is correct.