The working of a rocket is based on the principle of

Concept:

Law of conservation of momentum

• When two or more objects colloid, total momentum remains conserved. 
• This is based on the second law of motion which says force applied is proportional to the 
• If two masses m1 and m2 move with initial velocity u1 and u2, colloid and if the final velocity of m₁ becomes v1 and final velocity of m2 becomes v2 then 

m1u1 + m2u2 = m1v1 + m2v2

Explanation:

• Rocket: A rocket works by expelling gases from one end at a very high velocity.
• The escaping gas has a very high speed and this with its mass translates to a very large momentum. due to the principle of conservation of momentum the body of the rocket is pushed forward.
• The net momentum of the rocket and gas remains conserved as no external force is acting on the system of rocket and gas.
• So in order to keep net momentum zero, gas applies force on the rocket and the rocket moves up.
• This is also according to third law of motion that says every action has an equal and opposite reaction. 

Law of conservation of Momentum is the correct option

Additional Information

• Keepler gave three laws of planetary motion. 
• Newton's Law of gravitation states about the gravitational force between any two masses. 
• Elasticity: It is the property of regaining its original shape.