Let's try to get home quickly before the rain really _______ .  sets on  sets off  sets in  sets down

To fill in the blank correctly in the sentence, you need to choose an option that conveys the idea of rain starting or becoming more intense.

1. Sets on - This phrase is not commonly used in English.
2. Sets off - This generally means to start or trigger something but is not used in the context of rain.
3. Sets in - This phrase means to begin, especially in terms of weather. This is the most appropriate choice here.
4. Sets down - This does not make sense in this context.

After evaluating the options, the correct phrase to complete the sentence is "sets in."

Therefore, the completed sentence reads: "Let's try to get home quickly before the rain really sets in."