When is the stop and start system best employed?

The stop and start system is most effectively employed when the air demand is low and infrequently needed. This approach allows for energy savings and operational efficiency because the system can be turned off during periods of low demand rather than running continuously. This is particularly advantageous in scenarios where compressed air is not needed constantly, allowing the system to conserve energy and reduce wear on the equipment.

In contrast, employing this system when air demand is constant would lead to inefficient operation, as the compressor would need to cycle on and off frequently, potentially leading to excessive wear and greater energy consumption. While a large AC motor or a diesel engine can provide power, the stop and start mechanism is not inherently linked to the type of motor; rather, it is more suited to variable demand scenarios like low and infrequent air requirements. This is why B is the most suitable choice for the application of a stop and start system.