What does 'to be in business of' mean?

[Omaro]

Business Idiom in English, Intermediate level

ESL/EFL Test #53 "Business Expressions (2)", question 2

That's really not my concern at all and I'm certainly not ......... the business of telling others what to do.

(a) in
(b) out
(c) of
(d) by

Business Idiom in English, Intermediate level

ESL/EFL Test #53 "Business Expressions (2)", answer 2

That's really not my concern at all and I'm certainly not in the business of telling others what to do.

Correct answer: (a) in
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Hello Torsten

Can you help me this sentence, please? Which one can I choose and why?
Thank you very much
Omaro

[Mister Micawber]

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You can choose any of them, but you will be wrong unless you choose '(a) in'. 'To be in the business of' is an idiom used to indicate one's rightful responsiblility.
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Native English teacher at Mister Micawber's