In a normally aspirated engine, what happens to exhaust back pressure as an aircraft climbs?

In a normally aspirated engine, as an aircraft climbs, the atmospheric pressure decreases. This reduction in ambient pressure results in lower density of the intake air, which ultimately affects the engine's performance. As the aircraft climbs higher, the exhaust back pressure typically decreases due to the reduced air pressure in the atmosphere.

When the exhaust back pressure decreases, it allows the engine to expel exhaust gases more efficiently. This improvement in exhaust flow can help enhance engine performance to some degree, although normally aspirated engines will still experience a decline in power output with a loss of intake air density. By reducing the back pressure, there is less resistance for the exhaust gases, which can contribute to a slower rate of decline in engine power output as the aircraft ascends. This is why the choice correctly identifies that the decrease in exhaust back pressure can reduce the rate of decline in engine power output during a climb.

Other options present scenarios that do not accurately reflect the effects of climbing on exhaust back pressure or engine performance. For instance, an increase in back pressure would not occur during a climb and would not correlate positively with engine power output. Similarly, variations in power lever position do not directly affect exhaust back pressure in the context of altitude changes.