For the end point analysis of F, the acting force of F moves in the direction of F. From the meaning of the question, when the object moves L to the right, the acting point of the force moves L in the direction of the force and also moves L to the right with the pulley. Then the combined displacement of the action point of the force is shown in the figure, and L is equal to 2Scos θ 2.

Then f does work on the object W=FL, cos θ 2 = 2fs cos θ 2.

According to mathematical knowledge, 2fs cos2θ 2 = fs (1+cosθ);

So: CD.