What does 'knock on' mean here?

[Itsybitsyspider]

English Language Tests, Intermediate level

ESL/EFL Test #186 "Phrasal idioms with the verb 'knock'", question 4

A few months ago, James got a ......... his front door.

(a) knock off
(b) knock on
(c) knock out
(d) knock up

English Language Tests, Intermediate level

ESL/EFL Test #186 "Phrasal idioms with the verb 'knock'", answer 4

A few months ago, James got a knock on his front door.

Correct answer: (b) knock on

Your answer was: incorrect
A few months ago, James got a knock off his front door.
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What does 'knock on' mean here?

Thanks in advance.

[Alan]

Hi,

This means that someone was at his front door and wanted him to come to the front door. If he had had a bell, the person would have rung the bell.

Alan
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English as a Second Language
You can read my ESL story Womens' Day