Is "run-in" informal?

[Zarinamo]

English Language Tests, Intermediate level

ESL/EFL Test #150 "Idioms with the phrasal verb run", question 10

My mum had a ......... with my teacher for not allowing me to go to the toilet during class.

(a) run-on
(b) run-in
(c) rundown
(d) run-off

English Language Tests, Intermediate level

ESL/EFL Test #150 "Idioms with the phrasal verb run", answer 10

My mum had a run-in with my teacher for not allowing me to go to the toilet during class.

Correct answer: (b) run-in

Your answer was: correct
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Is thу phrase "run-in" formal???
When can we use it???
Thank you...

[Torsten]

Yes, it's an informal or colloquial term that means 'quarrel' or 'argument'.
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