Final answer:
Inertia is not a force, but rather the property of an object to maintain its state of motion. The correct answer is not given in the options provided, as inertial force is a misnomer and does not represent an actual force like gravitational, centripetal, or frictional forces, all acting externally on objects. Hence, the correct answer is option (C).
Step-by-step explanation:
Objects tend to stay in motion due to a property called inertia. Inertia is not a force; instead, an object tends to maintain its state of motion unless acted upon by an external force. The options given in the question refer to different types of forces which are not synonymous with inertia.
Specifically, inertia relates to Newton's First Law of Motion, which states that an object will remain at rest or in uniform motion in a straight line unless acted upon by a net external force. Therefore, the correct answer is not listed among the provided options, as inertial force is not a real force but rather a property of matter. The types of forces mentioned like gravitational force, centripetal force, and frictional force all act on objects but do not constitute inertia itself.
Centripetal force is the "center-seeking" force necessary for an object to maintain uniform circular motion. It always points toward the center of the path. Without this force, an object would move in a straight line of the circular path due to its inertia. While frictional and gravitational forces can affect an object's motion by opposing or attracting it, they are also not the same as inertia.