The correct answer is option 3: Linear momentum.

Linear momentum is a fundamental concept in physics that describes the motion of an object. It is defined as the product of an object`s mass and its velocity. According to Newton`s third law of motion, for every action, there is an equal and opposite reaction. This means that when a rocket emits gas or propellant at a high velocity, it experiences an equal and opposite force that propels it forward.

When the propellant is expelled from the rocket at a high velocity, it carries a certain momentum in the opposite direction. As per the law of conservation of linear momentum, the total momentum of the system (rocket + expelled gas) remains constant. As a result, the rocket gains momentum in the opposite direction, causing it to move forward.

Options 1, 2, and 4 are not directly related to the functioning of a rocket. Mass (option 1) is a property of matter and affects the acceleration of the rocket, but it is not a conservative law. Energy (option 2) is also important for the rocket`s functioning, but it is not specifically a conservative law. Angular momentum (option 4) deals with rotational motion, which is not the primary mechanism of a rocket`s propulsion