Explanation: The I.F. (Intermediate Frequency) amplifier bandwidth is critical for a receiver's selectivity and ability to resolve different types of signals. * **Narrow bandwidths** are essential for receiving weak signals (often associated with "long range" or DX communication) because they improve the signal-to-noise ratio (SNR) by filtering out more noise and interference. This is crucial for modes like Morse code (CW) and single-sideband (SSB) voice, which occupy narrow portions of the spectrum. * **Wide bandwidths** are necessary for signals that occupy more frequency space, such as FM voice. While a wider bandwidth passes more noise, it's acceptable and often required for strong, local signals (associated with "short range" communication) where fidelity is important or the signal itself is inherently broadband. Therefore, for optimal performance across various communication modes and signal conditions, a receiver's I.F. bandwidth needs to be adjustable: wide for local, strong signals and narrow for weak, distant signals. * **B is incorrect** because a wide bandwidth would worsen the SNR for weak, long-range signals. * **C is incorrect** because different communication modes require vastly different bandwidths. * **D is often true for modern radios** but describes the *mechanism* of adjustment, not the *reason* for needing different bandwidths, which option A explains.