The correct answer is A. inertia forces.
Inertia forces are the forces that resist any change in the motion of an object. They are proportional to the mass of the object and the acceleration of the object.
Internal forces are the forces that act between the parts of a system. They are caused by the interaction of the particles that make up the system.
Stored energy is the energy that is stored in a system due to its configuration or motion. It can be in the form of potential energy or kinetic energy.
Friction is the force that opposes the relative motion of two surfaces in contact. It is caused by the roughness of the surfaces and the adhesion between them.
The transient response of a system is the response of the system to a sudden change in the input. It is mainly due to inertia forces because inertia forces resist any change in the motion of the system. The other options are not the main cause of the transient response of a system.