The Principal's advice will prove salutary for the students.

Answer: Option C

Encouraging : giving someone support or confidence; supportive.
Remarkable : worthy of attention; striking.
Beneficial : resulting in good; favourable or advantageous.
Harmful : causing or likely to cause harm.
Salutary : (especially with reference to something unwelcome or unpleasant) producing good effects; beneficial.

Synonym of Salutary is Beneficial