Which of the following would increase the total flight time given a fixed amount of fuel?

When considering factors that can increase total flight time given a fixed amount of fuel, lower RPM settings stand out as a key element. Operating at lower RPMs generally results in reduced power consumption, which allows the aircraft to use fuel more efficiently. This adjustment enables the aircraft to maintain flight for a longer duration without reaching fuel depletion.

Lower altitudes can actually lead to increased drag and may result in a decrease in efficiency, which would counteract potential flight time gains. Higher temperatures can affect engine performance and fuel efficiency negatively, leading to increased fuel burn. Similarly, an increase in payload can result in greater weight, which typically requires more power to maintain flight, thus consuming fuel at a higher rate.

In contrast, lower RPM settings effectively extend the aircraft's endurance by optimizing fuel usage for a given power output, thereby increasing total flight time under fixed fuel conditions.