What does this phrase mean: "I don't care"? Is it an idiom?

[Guest]

Test No. incompl/elem-28 "Responses", question 9

Mike: 'You know the boss will be angry with you.'
Jane: '.........'

(a) I'm not caring.
(b) I'm not at all caring.
(c) I care nothing.
(d) I don't care.

Test No. incompl/elem-28 "Responses", answer 9

Mike: 'You know the boss will be angry with you.'
Jane: 'I don't care.'

Correct answer: (d) I don't care.
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hi,
could u tell me why cant we say"I care nothing"

[Torsten]

Because the correct phrase is I don't care.
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