Final answer:
The rate of detraining when reducing training days from four to two per week is most affected by Training frequency. To counteract detraining, intensity may be increased, but the key factor is maintaining a consistent frequency of exercise.
Step-by-step explanation:
What most affects the rate of detraining when switching from 4 days to two days of training per week is D. Training frequency. Detraining occurs when there is a significant reduction in training volume or frequency, leading to a reversal of fitness gains. When reducing training days from four to two, the body has more time to regress due to insufficient stimulus to maintain previous conditioning levels. To mitigate detraining effects, individuals may increase the intensity of each session, though other factors like nutrition, sleep patterns, and exercise type do play roles in overall fitness, the frequency of exercise sessions is directly related to how quickly detraining can occur.