The cool air from which component passes through the heat exchanger in flight?

The ram air inlet is the correct choice because it is specifically designed to collect outside air during flight, especially when the aircraft is in motion. This intake allows ambient air to flow into the system at relatively high speeds, taking advantage of the aircraft's forward motion.

In the context of the heat exchanger, the cool ram air that enters through this component is utilized to cool various systems within the aircraft, such as the environmental control system. As the ram air passes through the heat exchanger, it absorbs heat from the warmer air or fluid circulating within the aircraft, thus effectively cooling it.

Other options might serve different functions within the aircraft's air system, but they do not fit the context of cool air passing through the heat exchanger specifically during flight. The fresh air inlet, for example, is more about bringing in unconditioned air but does not operate with the same efficiency or purpose as ram air in the context of cooling through a heat exchanger.

In summary, the ram air inlet effectively channels cool air into the system, making it essential for maintaining optimal temperatures within the aircraft systems during flight.