The correct option is B calm down
A phrasal verb is a verb combined with an adverb or preposition to
provide a meaning different from the literal meaning of that individual
verb. The underlined phrase 'calm yourself' is not a phrasal verb, as it's of the combination verb (calm) + pronoun (yourself), hence it isn't the correct phrase and needs to be changed. 'Calm down' is a phrasal verb that means 'to feel more relaxed and less stressed', and is relevant to the given statement, hence option B is correct and A is wrong. Similarly, 'calm him' and 'cool yourself' are not phrasal verbs, 'him' and 'yourself' are pronouns and not prepositions/adverbs, hence these phrases cannot be used and options C and D are incorrect.