Fermentation produces much less usable energy per molecule consumed than aerobic respiration, so if it were the only way for roots to obtain energy, they would have less energy for growth in a given time. Poor aeration would mean little oxygen for the hydroponically-grown roots, making them rely on fermentation.
Work Step by Step
Review the material on fermentation in Ch. 9. Then you need to make the connection that poor aeration would provide little oxygen and limit cellular respiration. This would make cells rely on fermentation.