Meaning of "iron out"

[Svitlana_Morokhovych]

English Language Tests, Intermediate level

ESL/EFL Test #194 "Phrasal Verbs (H to L)", question 2

We still have to ......... the finer details about our upcoming trip to Scotland.

(a) iron in
(b) iron on
(c) iron out

English Language Tests, Intermediate level

ESL/EFL Test #194 "Phrasal Verbs (H to L)", answer 2

We still have to iron out the finer details about our upcoming trip to Scotland.

Correct answer: (c) iron out

Your answer was: incorrect
We still have to iron in the finer details about our upcoming trip to Scotland.
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What does it mean "iron out"?

[Mister Micawber]

Correct; perfect; eliminate any remaining minor problems
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Native English teacher at Mister Micawber's