Phrase: "What are you on about?"

[Liania]

English Language Tests, Intermediate level

ESL/EFL Test #211 "What Comes Next? (5)", question 5

You look a little confused. Do you know .........?

(a) what I'm on about
(b) what I'm off about
(c) what I'm under about
(d) what I'm in about

English Language Tests, Intermediate level

ESL/EFL Test #211 "What Comes Next? (5)", answer 5

You look a little confused. Do you know what I'm on about?

Correct answer: (a) what I'm on about
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I don't know what your asking

[Mister Micawber]

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We are asking which phrase makes sense in the blank. Of the four answers (a,b,c,d), only what I'm on about makes sense-- to be on about is an idiom meaning to be talking about intensely or at length. The other phrases have no meaning; their prepositions are wrong.
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Native English teacher at Mister Micawber's