meaning of the phrase 'on purpose'

[Illy]

English Grammar Tests, Elementary Level

ESL/EFL Test #97 "Common English phrases with prepositions", question 5

You're really annoying me. You're doing it ......... purpose, aren't you?

(a) about
(b) of
(c) on
(d) in

English Grammar Tests, Elementary Level

ESL/EFL Test #97 "Common English phrases with prepositions", answer 5

You're really annoying me. You're doing it on purpose, aren't you?

Correct answer: (c) on
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i don't know what is the answer. can you explain me about this?

Illy

[Lost_Soul]

Hi,
First I'd like to correct your question

[Conchita]

As you can see above, the right preposition here is 'on'. The phrase 'on purpose' means 'intentionally' or 'according to what we had planned'.