Based on the guideline $1$ provided which of the following is NOT recommended when designing a relational database schema?

Question $28$ options:

Each tuple in a relation should represent one entity or relationship instance.

Attributes of different entities $($e$.$g$.,$ EMPLOYEEs, DEPARTMENTs, PROJECTs$)$ should not be mixed in the same relation.

Only foreign keys should be used to refer to other entities.

Entity and relationship attributes should be combined in the same relation as much as possible.