The correct answer is: A. capacity/inflow ratio.
The trap efficiency of a reservoir is the fraction of sediment that is trapped by the reservoir. It is a function of the reservoir’s capacity and inflow. The higher the capacity of the reservoir, the more sediment it can trap. The higher the inflow, the more sediment is transported to the reservoir. Therefore, the trap efficiency of a reservoir is a function of the capacity/inflow ratio.
Option B is incorrect because the outflow of a reservoir is not a factor in the trap efficiency. The outflow of a reservoir is the amount of water that flows out of the reservoir. It is not related to the amount of sediment that is trapped by the reservoir.
Option C is incorrect because the outflow/inflow ratio is not a factor in the trap efficiency. The outflow/inflow ratio is the ratio of the outflow to the inflow. It is not related to the amount of sediment that is trapped by the reservoir.
Option D is incorrect because the trap efficiency is a function of the capacity/inflow ratio.