Step 1: Define the state being asked about.
"Mentally healthy" means well-adjusted and emotionally stable, not simply "free of illness."
Step 2: Link the definition to mood.
A well-adjusted, emotionally stable child naturally shows this through a cheerful and settled mood most of the time.
Step 3: Rule out the negative options.
Irritation, sadness, and loneliness are the kind of signs a teacher or parent watches for as warning signs, not as markers of good mental health.
Final Answer:
Happiness is the outward sign of good mental health here.