What is the correct pronoun to complete the sentence: "__________ of these train lines will take me to the airport in the shortest time possible?"

The correct pronoun to complete the sentence is "which." In this context, "which" is used to specify a particular subset from a group—in this case, the train lines. The phrase implies that there is a selection among different train lines, and the speaker is trying to identify exactly which one leads to the airport in the shortest time.

Using "which" effectively narrows down the options and invites a response that addresses the specific alternatives available, making it the appropriate choice for this sentence. The other pronouns do not fit the grammatical or contextual needs of the sentence. For instance, "what" is more general and would not adequately point to a defined selection; "who" refers to people, which is irrelevant here; and "whose" indicates possession, which does not apply to train lines in this context.