Well We first need to consider our answer choices.
They give us the F.I.T.T. principles.
We know what each stands for as well.
F = Frequency: How often you exercise
I = Intensity: How hard you work during exercise.
T = Time: How long you exercise
T = Type: what type of activity your doing.
In this case, since he is adding an extra day of training to his routine.
The answer would be Frequency.
Good Luck! :)