If no external force applied then acceleration will be zero ,so velocity remain conserve . Momentum=mass x velocity if velocity don't change momentum also not change.

The correct answer is option B: momentum.

Explanation:

Conservation of momentum is a fundamental concept in physics. It states that the total momentum of a system remains constant when no external force is acting on it.

Momentum is defined as the product of an object's mass and its velocity. Mathematically, momentum (p) can be expressed as:

p = m * v

where m is the mass of the object and v is its velocity.

When no external force acts on an object, its velocity remains constant. This means that its momentum also remains constant because the mass of the object does not change.

To understand why momentum is conserved when no external force is acting, we need to consider Newton's second law of motion. According to this law, the rate of change of momentum of an object is equal to the net force acting on it. In other words:

F = Δp / Δt

where F is the net force, Δp is the change in momentum, and Δt is the time interval over which the change occurs.

When no external force acts on an object, the net force is zero. Therefore, the change in momentum is also zero. This means that the momentum of the object remains constant.

Conservation of momentum has many practical applications. For example, it explains why a moving car takes longer to stop than a stationary car. When the brakes are applied to a moving car, an external force is acting on it, causing a change in momentum and eventually bringing the car to a stop. On the other hand, a stationary car does not experience any external force, so its momentum remains constant and it does not move.

In conclusion, when no external force acts on an object, the physical quantity that remains conserved is momentum.