Explanation: An FM ratio detector uses two diodes and a specialized circuit called the "ratio arm" to convert frequency variations into amplitude variations, and then into audio. A key advantage of the ratio detector over other FM discriminators (like the Foster-Seeley) is its inherent ability to reject amplitude modulation (AM) noise. This AM rejection capability comes from the ratio arm, which includes a large capacitor that charges to a voltage proportional to the average amplitude of the input IF signal. This voltage then stabilizes the detector's operating point, suppressing any amplitude variations (which include noise). If the ratio arm is not perfectly balanced, its ability to reject AM noise is compromised. Noise riding on the FM signal's amplitude will be incorrectly converted into audio, degrading the signal-to-noise ratio (SNR). An adjustment to a "Ratio Arm balance control bridge" (often a potentiometer) allows for fine-tuning the symmetry of the two diode paths within the detector. By achieving optimal balance, the detector can most effectively cancel out amplitude variations, thereby improving its AM rejection and, consequently, the overall signal-to-noise ratio. * **A) The diode detector:** The diodes themselves are fixed components; you don't adjust them. * **B) Input signal to the two diodes:** The input signal level is determined by preceding IF stages, often under AGC control, not an internal adjustment of the detector for SNR. * **C) Input voltage to the transistor:** A ratio detector is typically a passive diode circuit. While a receiver has transistors, this option doesn't refer to an adjustment *within* the ratio detector itself for SNR.