What does this expression mean: 'hard at it'?

[ryder]

Test No. errors/elem-7 "Prepositions", question 6

Charlie is always on it from morning till night - he's probably the hardest worker I've ever met.

(a) on
(b) from
(c) till

Test No. errors/elem-7 "Prepositions", answer 6

Charlie is always at it from morning till night - he's probably the hardest worker I've ever met.

Correct entry: at
The error was: (a) on

You have found the error and your entry is correct.
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"be at it" means "be busy"?

[Alan]

Yes, this means busy - very active and usually working hard. It is often used as this expression hard at it.
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