Fun vs. a joke

[Magomed]

Test No. incompl/inter-28 "The Hare and the Tortoise", question 2

He also made ......... of the slow pace at which the tortoise travelled.

(a) fun
(b) joke
(c) laugh
(d) funny

Test No. incompl/inter-28 "The Hare and the Tortoise", answer 2

He also made fun of the slow pace at which the tortoise travelled.

Correct answer: (a) fun

Your answer was: incorrect
He also made joke of the slow pace at which the tortoise travelled.
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Why word joke doesn't fit in here?

[Alan]

Hi,

In this sentence you would have to say A joke.

Alan
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English as a Second Language
You can read my ESL story Present Simple

[Watie]

These kind of mistakes make us slip to learn english.

Thank You!