The correct answer is D. Constant.
A recorder is said to have good frequency response when the sensitivity of the system is constant for all frequencies present in the signal. This means that the recorder will produce an output that is proportional to the input, regardless of the frequency of the input signal.
If the sensitivity of the recorder is not constant, then the output will be distorted. For example, if the sensitivity is higher at high frequencies than at low frequencies, then the output will be amplified at high frequencies and attenuated at low frequencies. This will result in a signal that is not faithful to the original input signal.
It is important for a recorder to have good frequency response in order to accurately record and reproduce signals. This is especially important for signals that contain a wide range of frequencies, such as music or speech.