If I _________________ you, I would leave now.

The selected answer "were" is correct because this sentence employs the subjunctive mood, which is used to express hypothetical or contrary-to-fact situations. In English, when discussing situations that are not real or are hypothetical, the subjunctive form "were" is used with the plural subject "I," rather than the indicative "was."

In this context, the speaker is suggesting a condition that is not currently true—for instance, they are not actually the other person and are merely imagining what they would do if they were. This grammatical structure emphasizes the unreality of the situation.

Using "was" would not fit because it does not conform to the subjunctive mood required for this kind of hypothetical statement. "Where" is incorrect as it indicates a location, which is not relevant in this context. Likewise, "which" is a relative pronoun used to provide additional information but does not serve any purpose in this construction regarding a hypothetical action. Thus, "were" appropriately conveys the intended meaning and grammatical correctness of the sentence.