The correct answer is “will go.”
The future tense is used to talk about things that will happen in the future. In this case, the speaker is warning the listener that they will get drenched if they go outside. The most appropriate way to express this warning is to use the simple future tense, which is formed with the verb “will” followed by the infinitive form of the verb.
The other options are not as appropriate because they do not express the same level of certainty. “Are going” is the present continuous tense, which is used to talk about things that are happening now or that will happen soon. “Shall go” is the formal future tense, which is rarely used in everyday speech. “Go” is the simple present tense, which is used to talk about things that happen regularly or habitually.
In conclusion, the correct answer is “will go” because it is the most appropriate way to express the speaker’s warning that the listener will get drenched if they go outside.