What factor contributes to the expansion of bread in the proofer?

The correct answer highlights the role of humidity in the expansion of bread during the proofing process. When bread dough is placed in a proofer, maintaining an optimal level of humidity is crucial for yeast activity and fermentation. Humidity helps to keep the surface of the dough moist, preventing it from forming a crust that could inhibit expansion. This moisture allows the yeast to work effectively, creating carbon dioxide gas, which causes the dough to rise and expands its volume.

While temperature is also important for fermentation, it alone does not directly facilitate the dough's expansion as humidity does. An ideal environment requires both factors, but in the context of the question, focusing specifically on humidity provides insight into one of the critical elements that directly contribute to the dough's ability to rise and achieve the desired texture.