Explanation: In Frequency Modulation (FM), the modulation index is the ratio of the frequency deviation to the modulating audio frequency. A modulation index set to less than 1 indicates that the maximum frequency deviation of the carrier is relatively small compared to the frequency of the modulating audio signal. Since the amount of frequency deviation in FM is directly proportional to the amplitude (level) of the modulating audio signal, a low modulation index implies that the audio input level to the transmitter is insufficient or too low. This results in a transmitted signal that sounds quiet or has a low perceived audio level to a receiving station, which is described as "unusually low level transmit audio." Excessive deviation (A) would occur with a high modulation index or too much audio input. Unstable signal excursions (B) are generally indicative of circuit problems, not simply a low modulation index. An unusually high pitch audio response (D) relates to the frequency content of the audio, not the overall modulation level determined by the modulation index.