What does "plum job" mean?

English as a Second Language English Teacher Explanations (ESL Tests)
1

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

(a) pear
(b) banana
© lemon
(d) plum

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

Correct answer: (d) plum

Your answer was: incorrect

Could you explain to me,please:
What does “plum” mean in this case?
What is “plum job?”

Thanks in advance

2

A “plum job” is a very easy, pleasant job that pays well. Sometimes it means a very good job that most people can’t get.