The members of this collection are January, June and July. Clearly, this collection is well defined and hence, it is a set.

A writer may be most talented for one person and may not be so for the other. Therefore, we cannot judge which writers are there in the collection. So, this collection is not well defined. Hence, it is not a set.

A cricketer may be best to one person and may not be to the other. Therefore, it cannot be said that which cricketer will be the best and hence this, collection is not well defined.
∴ It is not a set.
The term most dangerous is not a clear term. An animal may be most dangerous to one person and may not be for the other. So, it is not well defined, hence it is not a set.

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