What does stress out mean?

[Quynhg]

English Language Tests, Intermediate level

ESL/EFL Test #858 "Stress (1)", question 6

What circumstances stress you ......... the most?

(a) on
(b) in
(c) out
(d) along

English Language Tests, Intermediate level

ESL/EFL Test #858 "Stress (1)", answer 6

What circumstances stress you out the most?

Correct answer: (c) out

Your answer was: incorrect
What circumstances stress you in the most?
_________________________

please tell me why "out" not "in"?

Quynh, Vietnam

[Beeesneees]

To be 'stressed out' is a set phrase. If you are 'stressed out' you are understood to be suffering from a high level of stress. It might be so high that it begins to have a negative impact on your health and well-being.

There is no equivalent phrase 'stressed in', so it doesn't make sense in a sentence.
_________________
"Inside every old person is a young person wondering what happened."
Terry Pratchett