What does 'plum job' mean? It's an idiom, isn't it?

[Heiner]

Test No. incompl/inter-4 "Easy Questions", question 4

He is very lucky to have got himself a ......... job.

(a) pear
(b) banana
(c) lemon
(d) plum

Test No. incompl/inter-4 "Easy Questions", answer 4

He is very lucky to have got himself a plum job.

Correct answer: (d) plum

Your answer was: incorrect
He is very lucky to have got himself a pear job.
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What does plum job mean?

[Alan]

This is a very good job - the sort of job everybody would like to have.
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English as a Second Language
You can read my ESL story Passive Voice