Hint: In physics, friction is a phenomenon when one body tends to move on a surface then surface tends to oppose its motion and this force due to surface in opposite direction to that of motion is called frictional force, it only acts when two bodies are in contact whether they are in rest or in motion.
Complete answer: Let us first understand meaning of self-adjusting force in brief: Self-adjusting force means it’s a self-adjusted force which means it adjusted its magnitude by itself in a way such that it doesn’t change the state of body which means if it’s in motion it tries to keep body in motion or a body in rest it tries to keep body in rest.
Let us suppose an external force is applied to a body which is at rest. While the body at rest it’s affected by static friction with the surface as external force tries to change the state of body from rest to motion, static friction adjusts its magnitude in a way such that it matches with external force magnitude and no motion takes place in the body. So static friction is considered as a self-adjusting force.
Hence, Static friction is a self-adjusting force.
Note:Remember, when external force is just greater than the threshold magnitude of static friction, static friction can’t adjust more and external force becomes greater than static friction which changes the state of body from rest to motion and static friction converts into Kinetic friction.