Explanation: An antenna tuner, more precisely known as an antenna impedance matching network, is used to transform the electrical impedance of the antenna system to match the output impedance of the transceiver, typically 50 ohms. By "altering the electrical characteristics," the tuner ensures maximum power transfer from the transmitter to the antenna and minimizes reflected power (Standing Wave Ratio or SWR), which protects the final amplifier stage of the radio and improves transmission efficiency. Option B is incorrect because an antenna tuner operates purely electrically; it does not physically change the length of the antenna itself. Such alterations would require mechanical adjustments. Option C is also not entirely accurate. While achieving an impedance match often makes the antenna *appear* resonant to the transmitter, the tuner's primary function is general impedance transformation, not specifically making the antenna emulate a half-wave element. It can match various antenna types and lengths.