Which of the following is a self-adjusting force?

$A$ block of weight $W$ rests on a horizontal floor with coefficient of static friction $\mu$. It is desired to move the block by applying the minimum amount of force. The angle $\theta$ from the horizontal at which the force should be applied and the magnitude of the force $F$ are respectively:

Two masses $A$ and $B$ of $10\, kg$ and $5\, kg$ respectively are connected with a string passing over a frictionless pulley fixed at the corner of a table as shown. The coefficient of static friction of $A$ with the table is $0.2$. The minimum mass of $C$ that may be placed on $A$ to prevent it from moving is ........ $kg$.

$A$ box of mass $2 kg$ is placed on the roof of a car. The box would remain stationary until the car attains a maximum acceleration. The coefficient of static friction between the box and the roof of the car is $0.2$ and $g=10 ms^{-2}$. This maximum acceleration of the car, for the box to remain stationary, is: (in $ms^{-2}$)

$A$ $10\, kg$ block is placed on a horizontal surface as shown. If an external horizontal force $F = 50\, N$ is applied,find the friction force acting on the block. (Take $g = 10\, m/s^2$)

$A$ block of mass $M$ is placed on a rough surface with a coefficient of friction $\mu = 3$. If the applied force $F$ is $(4/5)$ of the minimum force required to just move the block,find the total force exerted by the ground on the block.