Answer:
Option B, when they are not busy farming
Step-by-step explanation:
In the given piece of article, Taizong want to teach people for war only when they are non practicing farming as depicted in the statement below
"Teach people military arts when they are free from farming in order to equip them with a sense of military decorum and morale"
Option A, C and D are incorrect because there is no mention of training people for war during spring time, peacetime and when there is no war.