Answer = 2) usted
Since it was one stranger, usted is used as it’s singular. If it was 2 or more, you use ustedes. Similar to tu being used when speaking to one friend, and vosotros being used when you’re speaking to more than one person.
The difference between usted/ustedes and tu/vosotros is one is used when you don’t know the person or you’re being polite. It’s sometimes called the polite form. And when using usted, you use the he/she/it conjugation of the verb with usted before it e.g., ‘you are smart’ in usted form = Usted es inteligente, but if it’s to a friend or someone you know, you would write Tu eres inteligente.
And similarly, ustedes uses the they conjugation with ustedes before, e.g., ‘you all eat pizza’ in the ustedes form = Ustedes comen pizza, but you would normally write in the vosotros form, Comeis pizza.
Basically:
-Ustedes is the plural of usted.
-Usted is used as the polite of tu.
-Usted takes the he/she/it conjugation.
-Ustedes is used as the polite of vosotros.
-Ustedes takes the they conjugation.