Explanation: The primary purpose of using a high first Intermediate Frequency (I.F.) in a superheterodyne receiver is to achieve superior image rejection. An image signal is an undesired signal that, when mixed with the local oscillator (LO) in the receiver's first mixer, produces the I.F. frequency. This image frequency is located at the LO frequency plus or minus the I.F. frequency. By choosing a high first I.F., the image frequency is pushed much further away from the desired signal frequency. This makes it significantly easier for the receiver's pre-selector (the RF bandpass filter before the mixer) to attenuate the image signal sufficiently before it reaches the mixer, preventing interference. Options B and C are incorrect because the I.F. frequency itself doesn't directly reduce intermodulation products or third harmonic signals; these are primarily addressed by using highly linear components and good circuit design. Option D is incorrect because while greater signal amplification might be *easier* to achieve at lower frequencies (due to fewer parasitic effects), the *primary purpose* of a low I.F. isn't simply greater amplification, but often narrower bandwidth and better selectivity with simpler filter designs.