Yeasts are unicellular microorganisms of the fungi kingdom that can digest sugars using both aerobic and anaerobic cellular respiration. In the below experiment, what by-product of this reaction causes the balloons to expand?

This question is incomplete as it does not contain the representation of the experiment, however, based on general understanding, the answer is:

Carbon dioxide (CO2)

Explanation:

Yeasts, as stated in this question, are living unicellular organisms that belong to the KINGDOM FUNGI. They employ both the aerobic (with oxygen) and anaerobic respiration (without oxygen) in breaking down sugars. However, both methods of cellular respiration gives off CARBON DIOXIDE (CO2) as a byproduct.

Therefore, when a yeast is placed in a balloon in accordance to this experiment and is allowed to perform respiration either aerobically or anaerobically, it gives off CO2 gas, which causes the balloon to expand.