The option that best completes the gap is "stops."
The sentence is in the present tense, and the use of "until" suggests that the action of leaving is dependent on a certain condition being met. In this case, the condition is the stopping of the rain.
The present simple tense "stops" is the most appropriate option here because it indicates a future action that is dependent on the current weather condition. The other options, "will stop," "stopped," and "has stopped," do not accurately convey this sense of a present condition affecting a future action.
Therefore, the correct sentence would be: "We won't leave until it stops raining."