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Explanation why first answer is wrong is not correct.
One side of the ManyToMany must be made the owner of the relationship by specifying a mappedBy argument.
mappedBy is required only if you wish to have bi-directional mapping. However it is possible to have 2 uni-directional @ManyToMany mappings resulting in two join tables.
First answer is wrong because the targetEntity attribute on items field should point to Items.class not Category.class.
You are right but I think the mistake in the option that you pointed was unintentional. The explanation is actually correct but misleading. It is correct because the situation presented is a bidirectional relationship and misleading because it doesn't make it clear that it applies only to bidirectional relationship.
I have updated the option and the explanation.
thank you for your feedback!