An example of mandatory emergency equipment in a commercial vehicle is:

Answer: C — supply of spare electrical fuses

In a commercial vehicle, certain emergency equipment must be carried to ensure safety and compliance with regulations. Mandatory emergency equipment includes items that are essential for addressing potential emergencies that may arise while on the road. The presence of spare electrical fuses is critical as they can help restore power to various electrical components of the vehicle in the event of a failure. This is particularly important for ensuring that essential safety features, like lights and signals, remain operational. When a fuse blows, having a supply of spare fuses allows the driver to quickly replace the faulty component and reduce the risk of further complications or accidents. While a fire extinguisher, first aid kit, and a reflective warning triangle are also important components of emergency preparedness in commercial vehicles, the requirement for spare electrical fuses specifically addresses the operational readiness of the vehicle itself, making it a mandatory piece of equipment. This necessity aligns with safety regulations that mandate drivers be equipped to handle common issues that could jeopardize their safety or that of others on the road.