Assertion: Static friction is a self-adjusting force.
Reason: The coefficient of friction does not depend upon the mass of the body.
A) Both assertion and reason are correct and the reason is the correct explanation for the assertion.
B) Both assertion and reason are correct and the reason is not the correct explanation for the assertion.
C) Assertion is correct but the reason is incorrect
D) Assertion is incorrect but the reason is correct

Hint: A body experiences static friction, when it is at rest or about to move. It is an opposing force; hence it depends on the force applied on the body, and the coefficient of friction is dimensionless quantity. It is a property of surfaces in contact.
Formula used:
\[\text{ }\!\!\mu\!\!\text{ =}\dfrac{\text{Frictional force}}{\text{Normal force}}\]

Complete step-by-step solution:
Friction is of two types: static friction and kinetic friction. Static friction is applied to the body at rest or the body which is about to move. Static friction opposes the movement of the body. Its value is equal to applied force till the body remains in a static position. i.e., it self-adjusted friction, its value according to the applied force.
Hence, the assertion is correct.
The coefficient of friction, $\mu$, is a measure of the amount of frictional force existing between two surfaces. When its value is low, it indicates that the force required for sliding to occur is less. When its value is the high force required is high.
The coefficient of friction is given by,
\[\text{ }\!\!\mu\!\!\text{ =}\dfrac{\text{Frictional force}}{\text{Normal force}}\]
It is the property of surfaces. Hence, the coefficient of friction does not depend
on the mass of the body. Hence the reason given is also correct.
Therefore, both assertion and reason are correct and the reason is not the correct explanation for the assertion.
Hence, The answer is option B.

Note: The coefficient of friction is a dimensionless quantity, which implies that the direction of the force shall not affect the coefficient of friction. Its value depends on the objects that are causing friction. Its value is normally between 0 and 1 but it can be greater than 1.