Explanation: A push-pull amplifier design effectively reduces even-order harmonics. It employs two active devices (transistors or vacuum tubes) that operate 180 degrees out of phase with each other. When their outputs are combined, the fundamental signals add constructively. Critically, the even-order harmonics (2nd, 4th, etc.) generated by each device are also 180 degrees out of phase relative to each other at the combining point, causing them to largely cancel out. Odd-order harmonics, however, remain in phase and combine constructively. In contrast: * **A) Push-push amplifier** actually *enhances* even-order harmonics (especially the second harmonic) and is typically used for frequency multiplication. * **B) Operating class C** is highly non-linear, conducting for less than 180 degrees, and *generates a significant amount of harmonics* due to its pulsed operation. * **D) Operating class AB** provides better linearity than Class C but does not inherently cancel even-order harmonics in a single-ended configuration; the push-pull topology is responsible for the cancellation.