A man pushes on a box at an angle of 30.0° with a force of 20.0 N and the box moves across the floor. Which of the following component of the force do you use to calculate work?

Not sure what the given options are, but the answer is the horizontal component. This is given by Force x cos(angle), or Fcos(θ), where θ is the angle. In this case that would be 20cos(30) = 17.32N