Determine the design angle  (0o ≤  ≤ 90o) between struts AB and AC so that the 400-lb horizontal force has a
component of 500 lb directed from A towards C. What is the
component of force acting along member AB? Take θ = 40o.

i believe the answer would be:

d. violin, clarinet, french horn, and tuba

the box does not move because you need to use a force greater than 300 n. you are only using 50 n. it needs to at least be 301 n for the box to move because then it is an unbalanced force and it will move the way that the greater force is pushing it. if you use only 300 n it will be a balanced force and will not move because the force you are exerting on the   box is the same as its force.

hope this !

im sorry if i went too in depth, i am going to include a link to a video on khan academy that will explain this.   hope that you like the video and that it explains more in detail to you.

that is the link.