Translation:Where is the intersection?
When you ask directions to a particular place use "the". Because it's a particular place. Here they need to be directed to one particular intersection / junction. If any example of a type of place would be OK than you could use "an". E.G. "which way to the hotel?" means the one specific hotel that both the speaker and the listener know is being referred to.
"where can I find a(n) hotel?" means the speaker can be directed to any hotel.
I am getting so frustrated by Duolingo's reluctance to accept 'junction' instead of 'intersection' as the British English alternative. I get it wrong every time and report it every time, as I simply don't remember to 'translate' to intersection, which is a word I know but is not used in this country.