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.
_________________________

"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.
_________________
English as a Second Language
You can read my ESL story Present Simple

[Colonelcy]

ABBY LINGVO x3
to be at it - плохо себя вести - misbehave
Those children are at it again, I'll have to stop them! — Эти дети опять плохо ведут себя. Пора это прекратить!
be at it (informal)
The apples keep disappearing from my garden. I suppose those children are at it again — У меня в саду кто-то рвет яблоки. Я думаю, что эти сорванцы снова принялись за старое
The brakes are at it again — Тормоза опять не действуют
The kids are at it again. Why are they always fighting? — Пацаны опять дерутся. Когда они только перестанут?!