In which type of exercise would fat reserves be used to create atp? a strenuous, short hike with many steep climbs a moderate, but long hike over gently rolling hills

The right answer is the second option.

ATP is synthesizes in glycolysis, and through citric acid cycle.

In case of glucopenia (lack of sugars, after a long-term excerice), fat mobilization from adipose tissue is observed, followed by hydrolysis to fatty acids and glycerol.

Glycerol is used as a precursor in glucose recycling via gluconeogenesis: this accelerated gluconeogenesis requires energy.

Fatty acids are oxidized by beta-oxidation, pass through L-carnitine through the mitochondrial membrane (carnitine molecular shuttle) and via AcCoA into the citric acid cycle and even bring more energy that glucose burning but in the long run.