The best answer is D.
Facilitated diffusion is a channel mediated diffusion and functions much like a bridge over a river that must raise and lower in order to allow boats to pass. When the bridge is lowered, boats cannot pass to the other side of the river.
In a similar manner, a gated channel often remains closed, not allowing substances into the cell until it receives a signal (like the binding of an ion) to open.
When this signal is received, the bridge (gate) opens, allowing the boats (substance) to pass through the bridge and into the other side of the river (cell membrane)
