la respuesta seria "va a" llover porque las demas te darian una oracion incorrecta que no se entenderia, ademas de que "voy" y "vamos" estariamos hablando de alguien no algo..
Espero haberte ayudado
The correct answer would be "va a" because the others would give you an incorrect sentence that wouldn't make sense. Besides "voy and "vamos" is talking about someone not something
Hope this helped
