The correct answer is C: An antenna that concentrates signals in one direction. A beam antenna is an antenna that concentrates signals in one direction, providing gain in the forward direction while reducing radiation in other directions. Beam antennas include Yagis, quads, and other directional designs. The 'beam' refers to the focused radiation pattern, like a searchlight beam. For amateur radio operators, beam antennas provide significant advantages: they increase effective radiated power in the desired direction, reduce interference from other directions, and improve signal-to-noise ratio. Understanding beam antennas helps when selecting antennas for DX work and interference reduction.