The answer key identifies explosive behaviour as the outward sign intended by this question.
Why B fits: Repeated, disproportionate outbursts can disrupt relationships, judgement, and classroom management.
Why the others are weaker: Digestive upset, fatigue, and worry can have many causes and are less specifically observable as disruptive behaviour.
Important qualification: No single sign is enough to diagnose a mental health condition. A teacher with persistent difficulties should receive appropriate support rather than be labelled because of one incident.