The correct answer is D. All of the mentioned.
The switching speed of a MOS device depends on the gate voltage above a threshold, the carrier mobility, and the length of the channel.
The gate voltage above a threshold is the minimum voltage that must be applied to the gate in order to turn on the device. The higher the gate voltage, the faster the device will switch.
The carrier mobility is a measure of how easily electrons or holes can move through the semiconductor material. The higher the carrier mobility, the faster the device will switch.
The length of the channel is the distance between the source and drain terminals. The shorter the channel, the faster the device will switch.
In conclusion, the switching speed of a MOS device depends on the gate voltage above a threshold, the carrier mobility, and the length of the channel.