The correct answer is C. Cell size.
Spectral efficiency is a measure of how much data can be transmitted over a given frequency band. It is important in wireless communication because it determines how many users can be supported in a given area.
Cell size is the area that can be served by a single base station. A smaller cell size means that more base stations are needed, but each base station can serve fewer users. This results in a higher spectral efficiency.
Vocoder is a device that converts speech into a digital signal. It is not an important factor in determining spectral efficiency because it does not affect the amount of data that can be transmitted.
Modulation technique is the method used to encode data onto a carrier wave. It is an important factor in determining spectral efficiency because it affects the bandwidth of the signal. A wider bandwidth means that more data can be transmitted, but it also means that the signal is more susceptible to interference.
Multiple access technique is the method used to share the same frequency band among multiple users. It is an important factor in determining spectral efficiency because it affects the amount of interference that each user experiences. A lower interference level means that more data can be transmitted.
In conclusion, the correct answer is C. Cell size.