The correct answer is: C. who
The sentence is about a boy
who came to see the speaker this morning. The boy is the subject of the sentence, and the verb is “came.” The correct pronoun to use in this sentence is “who,” because “who” is used to refer to people.The pronoun “that” can be used to refer to people or things, but it is more commonly used to refer to things. The pronoun “whom” is used to refer to people who are the objects of verbs or prepositions. In this sentence, the boy is the subject of the sentence, not the object of a verb or preposition, so “whom” is not the correct pronoun to use.
The answer “No improvement” is incorrect because the sentence is grammatically incorrect. The correct pronoun to use in this sentence is “who.”