What serves as a starter motor for engine start in the aircraft?

The correct answer highlights the role of the Auxiliary Power Unit (APU) in providing the necessary power for engine start. The APU is specifically designed to generate electrical power and supply bleed air for starting the engines, which makes it a crucial component in many aircraft systems. During the starting process, the APU can deliver both electrical and pneumatic energy, enabling the engines to begin operating efficiently.

In contrast, the main battery serves different functions, primarily powering essential avionics and aircraft systems when the engines are not running. The generators (GEN 1 and GEN 2) are typically involved in providing power after the engines are started but do not directly contribute to the initial engine start process. Therefore, the APU stands out as the dedicated source for initiating engine operations in this context.