"Portare" means to take or carry something from point A to point B. So with a person as a subject, you could use it for, say, taking a book to class or taking a present to a party, but not a road, because the road stays put. Otherwise it would be something like, "Which roads are they carrying to the city?" which wouldn't make any sense.
As I understand it:
It is the streets that are "taking/bringing"(=leading to) ... = the streets lead = THEY lead = portano
It will be "portano" no matter who and how many the streets lead. Because it is the streets that are doing "it".
If the question is "which (one) street bring (me or them) / lead to the city" then the verb "portare" will be in singular for "the street leads" / "it leads" = porta
According to Google translate the verb "portare" can mean variations of : carry bring lead wear move
Duolingo showed as a correction "which roads take you to the city?" when I translated the Italian sentence to "which roads take them to the city?". I'm ok with "which streets lead to the city?" in this page, but, like many others before me, I don't understand the correction I received at first and I would like someone to explain that. How could "portano" refers to the second person? This point was not explained yet. Thanks.