Explanation: A Class C amplifier is distinguished by its operational characteristics designed for high efficiency, primarily used in RF applications. Option B is correct because a Class C amplifier has its output present for less than 180 degrees of the input signal cycle. This means the active device (transistor or tube) conducts for only a small fraction of the input waveform, often 90 degrees or less. Option C is also correct because this short conduction angle is achieved by setting the amplifier's bias well beyond cutoff. "Beyond cutoff" means the quiescent operating point is chosen such that the device is normally off and requires a significant portion of the input signal to turn it on and conduct, thus producing an output for only a brief period. Therefore, both the narrow conduction angle and the bias setting well beyond cutoff are defining characteristics of a Class C amplifier, making D the correct answer. Option A describes a Class A amplifier, which conducts for the entire input cycle.