Which of the following conditions can lead to increased ester production in beer?

In brewing, ester production is significantly affected by the yeast strain used and the fermentation conditions. Under-pitched yeast refers to using less yeast than the fermentation requires, which can stress the yeast. When yeast is stressed, it can lead to increased production of esters as a byproduct of metabolism. Esters contribute fruity and floral flavors to beer, making them a desirable characteristic in styles such as ales.

Low fermentation temperatures typically reduce ester production because yeast becomes less active and slows down its metabolic processes. High aeration is more commonly associated with increased yeast health and could suppress ester production as yeast may prioritize growth over ester synthesis. Low gravity worts generally lead to less nutrient competition and can also minimize ester production.

Thus, under-pitched yeast is the correct condition that can significantly lead to increased ester production in beer due to the stress it places on the yeast during fermentation.