The correct answer is A. Spectral capacity.
Spectral capacity is a measure of how much data can be transmitted over a given frequency band. It is often expressed in bits per second per hertz (bps/Hz). A higher spectral capacity means that more data can be transmitted over the same frequency band, which can improve the performance of a wireless system.
Channel capacity is a measure of the maximum amount of data that can be transmitted over a given channel. It is often expressed in bits per second (bps). Channel capacity is determined by the characteristics of the channel, such as the bandwidth, noise level, and interference.
Carrier capacity is a measure of the maximum number of users that can be supported by a given carrier. It is often expressed in users per cell. Carrier capacity is determined by the characteristics of the carrier, such as the bandwidth, power, and antenna gain.
Radio capacity is a measure of the maximum number of users that can be supported by a given radio system. It is often expressed in users per area. Radio capacity is determined by the characteristics of the radio system, such as the bandwidth, power, and antenna gain.
In conclusion, spectral capacity is the best measure of spectrum efficiency of a wireless system.