Concept:A possessive noun shows ownership and is formed by adding an apostrophe and s to a singular noun.Explanation:The sentence means everyone wants to be the favourite of the school principal.So we need the possessive form of the whole phrase school principal.The correct singular possessive is school principal's.This gives: Everyone wants to be the school principal's favourite.Option B wrongly adds another possessive, and Option D is not possessive.Answer:C. school principal's