A box is pressed against a vertical wall by a force F which is directed horizontally. If the magnitude of F is the minimum required to hold the box at rest, what is its value

 F = mg /μ

Explanation:

For this exercise we define a coordinate system with the horizontal x axis and the vertical y axis, let's write the equilibrium equations for each axis

X axis

         F -N = 0

          F = N

Y axisy

         fr - W = 0

         fr = W                (2)

the force and touch have the expression

           fr = μ N

we substitute

          fr = μ F

we substitute in 2

         μ F = m g

          F = mg /μ

its c its is a mixture of gases

i think the answer is a, hope this is right, that would be my best guess.