We will ___ the game if we do not have Peter.

The correct answer is "lose." In this context, "lose" is the correct verb form that fits the future tense construct of the sentence. The phrase "We will lose the game" indicates an outcome that is dependent on the future action or condition of not having Peter.

The verb "lose" in this case functions as the base form used after the auxiliary verb "will," which is a common structure in English for forming the future tense. This structure helps convey a future possibility or event.

The other options do not fit the grammatical structure required by the sentence. "Loose" refers to something not tight, "lost" is the past tense of lose and does not work here as the sentence is discussing a future event, and "loosen" means to make something less tight, which does not apply in the context of playing a game.