Type letter SIS is exclusively used for which of the following applications?

The designation "SIS" stands for "Supplementary Insulation Synthetics," which indicates a specific type of cable designed for particular applications. It is primarily used in wiring for switchboards and switchgear due to its enhanced insulation properties, which are crucial in environments where electrical equipment is housed and requires reliable protection against potential faults or failures. Switchboards and switchgear often handle high voltages and loads, demanding materials that can endure such conditions while providing safety and durability.

The other choices outline applications that may not have the same stringent requirements for insulation or the environmental conditions that SIS is specifically designed to meet. Home electrical wiring might typically use different types of wires suited for residential needs. Underwater applications often require specialized cables meant to resist corrosion and withstand aquatic conditions, which is not the primary focus of SIS cables. Similarly, outdoor wiring can involve cables that need to be weather-resistant but may not specifically benefit from the unique performance characteristics of SIS. Thus, the context in which SIS cables are applied highlights why wiring for switchboards and switchgear is the most fitting application for this cable type.