What key feature enhances the operational capability of the C-5 APU?

The correct answer highlights the independence of the C-5 APU from external power sources. This independence is a crucial feature because it allows the aircraft to operate its systems and engine start without relying on ground support or external power units. This capability is particularly beneficial in remote locations or situations where ground power might not be readily available. As a result, the APU can provide electrical power and pneumatic pressure for air conditioning and engine starting while the aircraft is on the ground, ensuring operational readiness at all times.

In contrast, the other features listed, while they may be beneficial in different contexts, do not specifically enhance the operational capability of the C-5 APU in the same fundamental way. The generation of thrust is not the primary function of an APU, since its main role is to support ground operations rather than to propel the aircraft. While lightweight construction can contribute to overall aircraft efficiency, it is not a defining characteristic of the APU's operational capabilities. The use of renewable energy sources is not applicable to the APU, as it typically operates on conventional fuel types, focusing instead on reliability and efficiency in its power generation tasks.