According to the comments that states that WHICH is the underlined word, the answer is that WHICH is a relative pronoun (Option 1)
A relative pronoun is a pronoun that introduces a relative clause. The most common relative pronouns are who, whom, whose, which, that. And "which" is used when referring to animals, objects, situations. In this case, "which" refers to "the truck" (an object).
A relative clause is a group of words that includes a subject and a verb, is always part of a sentence (it doesn't stand alone) and gives us extra information of the sentence or adds certain meaning to it. A relative clause can be defining (if it's removed, the meaning of the sentence changes significantly) or non-defining (if it's removed, we lose some detail but the overall meaning of the sentence remains the same).
Therefore, according to the definition provided, we can conclude that "which sometimes stalls" is a non-defining relative clause, and therefore "WHICH" is a relative pronoun.
