The correct answer is option 4- Inertia. Inertia is a property of matter that causes an object to resist changes in its motion. It is directly related to Newton`s First Law of Motion, which states that an object at rest will remain at rest, and an object in motion will continue moving at a constant velocity, unless acted upon by an external force.
Option 1- Momentum is not the correct answer. Momentum is a property that is related to the mass and velocity of an object in motion. It is a vector quantity and is given by the equation momentum = mass x velocity.
Option 2- Elasticity is also not the correct answer. Elasticity is the property of a material that allows it to regain its shape after being deformed by an external force. It is not directly related to the velocity of a moving body.
Option 3- Surface tension is not the correct answer either. Surface tension is the property of a liquid that allows it to resist an external force and maintain a minimal surface area. It is not related to the velocity of a body.
In conclusion, the property that causes the velocity of a body to remain constant unless an external force is applied is inertia.