The answer is C. Channel depth.
In an n-channel MOSFET, the channel depth is the distance between the source and drain regions. The channel depth is typically much larger than the channel length and width, so it can be considered to be constant.
The channel length is the distance between the source and drain regions along the channel. The channel width is the distance between the source and drain regions perpendicular to the channel. The channel concentration is the number of charge carriers (electrons in an n-channel MOSFET) per unit volume of the channel.
The channel length and width are important parameters in determining the characteristics of an n-channel MOSFET. The channel length determines the on-resistance of the MOSFET, while the channel width determines the current that the MOSFET can handle. The channel concentration is also important, but it is not as critical as the channel length and width.
In conclusion, the channel depth is constant in an n-channel MOSFET. The channel length and width are important parameters in determining the characteristics of an n-channel MOSFET, while the channel concentration is also important, but it is not as critical as the channel length and width.