The correct answer is: Two.
An enhancement-type nMOS device is a type of field-effect transistor (FET) that is turned on when a positive voltage is applied to its gate. When the gate is turned on, it creates a channel of current flow between the source and drain terminals. This current flow can be used to discharge the base current of a transistor.
To improve the base current discharge, two enhancement-type nMOS devices can be added to the circuit. These devices will be connected in parallel with the base resistor. When the gate of one of the devices is turned on, it will create a channel of current flow that will bypass the base resistor. This will cause the base current to decrease, which will improve the switching speed of the transistor.
The other options are incorrect because they do not provide enough enhancement-type nMOS devices to improve the base current discharge. One enhancement-type nMOS device will not provide enough current flow to bypass the base resistor. Three enhancement-type nMOS devices will provide too much current flow, which could damage the transistor. Four enhancement-type nMOS devices will provide more than enough current flow to bypass the base resistor, but it is not necessary to use this many devices.