Explanation: Good bearing resolution, in the context of direction finding or accurately determining the horizontal angle to a signal source, relies on the antenna's ability to discriminate between signals arriving from closely spaced directions. A **narrow antenna beam in the horizontal plane** (azimuth) provides high directivity. This means the antenna is highly sensitive to signals coming from a very specific horizontal angle and much less sensitive to signals from other angles. When you rotate such an antenna, the signal strength will peak sharply when the antenna points directly at the source, allowing for a precise determination of the signal's bearing. Conversely: * **A high transmitter output reading** (A) relates to the power of the outgoing signal, affecting range or signal strength, not the precision of determining direction. * **A high duty cycle** (B) refers to the proportion of time a transmitter is active, which impacts average power and heating, but not directional resolution. * **A narrow antenna beam in the vertical plane** (C) helps with determining elevation angle or reducing ground reflections, but bearing is a horizontal measurement.