Which is NOT a source of air for engine start?

The correct choice identifies a source that is not typically used for starting the engines. The Power Transfer Unit (PTU) primarily functions to assist with hydraulic pressure transfer and does not supply bleed air necessary for engine starting. In contrast, ground carts are specifically designed to provide the required air pressure for starting engines. An auxiliary power unit (APU) is also a common source, supplying bleed air to facilitate the engine start process. Furthermore, another engine can provide bleed air if it is already running, allowing for engine starts in certain scenarios.

Understanding the purpose and function of each option helps clarify why the Power Transfer Unit is not a correct source for engine start, as its primary role is not related to supplying air for this operation.