The correct option is C ran into
A phrasal verb is a verb combined with an adverb or preposition (verb + preposition or verb + adverb) to
provide a meaning different from the literal meaning of that individual
verb. 'Ran into' is a phrasal verb, as it's of the form verb (ran) + preposition (into), it means 'to meet by chance', hence option B is correct. 'I ran' is a phrase made of of a pronoun (I) and a verb (ran), it's not of the same form as a phrasal verb, hence option A is incorrect. 'Old school' is a phrase consisting of an adjective 'old' and a noun 'school', it does not contain a verb, it isn't a phrasal verb hence option C is incorrect. 'Today' is an adverb that refers to the day the speaker said this sentence. It's not a phrasal verb hence option D is incorrect.