When the right end of the light rope moves from point B to the top of the straight rod, let the included angle between the two ropes be 2 θ. Take the pulley as the research object, analyze the stress situation and make the stress diagram as shown in figure 1. According to the equilibrium conditions, we can get:

2Fcosθ=mg

Get the tension of the rope:

F=mg2cosθ

Therefore, when the right end of the light rope moves from point B to the top of the straight rod, θ increases, cosθ decreases and F increases.

When the right end of the light rope moves from the top of the straight rod to point C, let the included angle between the two ropes be 2 α.

Let the total length of the rope be L and the distance between two straight rods be S, which is obtained by mathematical knowledge.

Sinα=SL, L and S are unchanged, then α is unchanged.

According to the equilibrium condition, the tension f of the two ropes remains unchanged.

Therefore, the tension in the rope changes first and then remains the same.

So choose: a.