Explanation: For AC generators to operate in parallel, their outputs must be precisely synchronized to prevent damaging circulating currents and ensure stable power sharing. 1. **Output voltages must be equal:** If the voltages differ, a circulating current will flow from the generator with the higher voltage to the one with the lower voltage. This leads to inefficient power transfer, heating, and can cause the lower voltage generator to act as a motor, consuming power rather than producing it. 2. **Frequencies must match:** A difference in frequency means the phase relationship between the generators is constantly changing. This results in rapidly fluctuating instantaneous voltage differences, causing large current surges, instability, and potential damage or tripping of protective devices. 3. **Phase rotation and relative phase positions must match:** For multi-phase systems, the sequence in which the phases peak (phase rotation) must be identical. A mismatch here is akin to a direct short circuit, causing severe damage. Even with matching rotation and frequency, the instantaneous phase angle (relative phase position) between corresponding phases must be precisely aligned (typically near zero degrees) to minimize instantaneous voltage differences, which is usually achieved using a synchroscope. Therefore, meeting all these conditions ensures smooth, stable parallel operation, making C the comprehensive and correct answer.