Meaning of "taking care of business"

[mohhad]

English Idioms and Expressions, Advanced Level

ESL/EFL Test #43 "American Slang Words (17)", question 4

I've been at the office a lot lately. I'm just taking care of business.

(a) having a good time
(b) doing what I have to do
(c) starting a new business

English Idioms and Expressions, Advanced Level

ESL/EFL Test #43 "American Slang Words (17)", answer 4

I've been at the office a lot lately. I'm just doing what I have to do.

Correct answer: (b) doing what I have to do
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i`d like to have your explanation

mohhad

[Mister Micawber]

taking care of business = doing what I have to do
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Native English teacher at Mister Micawber's