Final answer:
Without auto tap changing underload equipment, it is essential to install voltage regulators to maintain constant voltage levels. Voltage regulators adjust to compensate for fluctuations, providing stable power supply.
Step-by-step explanation:
When a power transformer lacks auto tap changing underload equipment, it is usually necessary to install voltage regulators. The purpose of a voltage regulator is to maintain a constant voltage level. Transformers without automatic tap changers cannot adjust their turn ratios under load conditions to regulate voltage. Therefore, an external voltage regulator compensates for voltage fluctuations due to load changes or other factors.
Without such regulation, the power supply might experience voltage sags or surges, leading to ineffective operation or potential damage to the connected systems. Thus, voltage regulators play a critical role in ensuring a stable voltage supply from transformers that do not have built-in tap changing capabilities.
The correct answer to the question would hence be c. voltage regulators.