A

Understanding Frictional Force:
Frictional force is the force that resists the motion of an object. It acts parallel to the surface in contact and depends on the nature of the surfaces as well as the normal force between them.
Step 1: Nature of Frictional Force
Friction is often considered self-adjustable because it can vary based on the applied force. For example, if you push an object, the frictional force increases up to a certain limit (the limiting force) and will match the applied force to prevent motion until that limit is reached. After this limit, kinetic friction takes over, and it is generally less than static friction.
Step 2: Examining Other Options
- Option B (Not self-adjustable) is incorrect because it contradicts the nature of friction.
- Option C (scalar quantity) is misleading since friction is a vector quantity, having both magnitude and direction.
- Option D (Equal to the limiting force) is only true at the point of impending motion and thus cannot be universally accepted as correct.
Conclusion
Since the frictional force can adjust itself up until it reaches its limiting value based on the applied force, the most accurate statement about frictional force is that it is self-adjustable. Therefore, the correct answer is A.