How do static friction oppose motion ? Value of coefficient of friction depend on which factors ?

A block of mass $m$ is pressed against a vertical surface by a spring of unstretched length $l$ . If the coefficient of friction between the block and the surface is $\mu $. Choose the correct statement.

A $1\,kg$ block is being pushed against a wall by a force $F = 75\,N$ as shown in the Figure. The coefficient of friction is $0.25.$ The magnitude of acceleration of the block is ........ $m/s^2$

A block of mass $m$ is stationary on a rough plane of mass $M$ inclined at an angle $\theta$ to the horizontal, while the whole set up is accelerating upwards at an acceleration $\alpha$. If the coefficient of friction between the block and the plane is $\mu$, then the force that the plane exerts on the block is

The force required just to move a body up an inclined plane is double the force required just to prevent the body from sliding down. If $\mu $ is the coefficient of friction, the inclination of plane to the horizontal is

Which of the following is self adjusting in nature?