A and B let the included angle between the rope and the horizontal direction be α, the pulling force of the left rope be T 1, and the pulling force of the right rope be T2, which is obtained according to orthogonal decomposition:

Vertical direction: F=T 1sinα+T2sinα, while T 1=Mg, T2=mg, ((M+m)gsinα). As the circle moves down, α increases, sinα increases, and then F increases.

Horizontal direction: the action of the rod on the ring is n = t1cosα-t2cosα = (m-m) gcosα. When α increases, cosα decreases, and then n decreases. So, AB is all wrong.

C, the ring is subjected to external force F, the action of the rod on the ring and the tension of two ropes. According to the equilibrium condition, the resultant force of the rod acting on the ring C and the external force F is equal to the resultant force of the two ropes, but the tension of the two ropes is constant, the included angle decreases and the resultant force increases, so the resultant force of the rod acting on the ring C and the external force F increases. So, C is correct.

D, the tension of the two ropes is constant, the included angle is changing, and the direction of their resultant force is also changing, so the direction of the resultant force of the rod on the ring C and the external force F is also changing. So, d is wrong.

So choose C.