Explanation: Even harmonics are a form of distortion caused by non-linear operation in an amplifier. **A) By operating the stage push-pull:** In a push-pull amplifier, two active devices operate 180 degrees out of phase. The non-linear characteristics of each device tend to produce both even and odd harmonics. However, due to the phase relationship, the *even* harmonics generated by one device are out of phase with the even harmonics from the other, causing them to cancel out in the output transformer or summing point. Odd harmonics, conversely, are in phase and add. **B) In a single tube amplifier, by operating the stage only on the linear part of the tube's characteristic curve:** Any amplifier operating outside its linear region introduces distortion. This distortion manifests as the creation of harmonic frequencies (multiples of the input frequency). By carefully biasing and driving a single-ended amplifier so that it operates strictly within the linear portion of its characteristic curve, the generation of *all* harmonics, including even ones, is significantly reduced, as the output signal closely mirrors the input. Since both methods are effective in reducing even harmonics, option C is the correct choice.