Informally, each tuple in a relation should represent one entity or relationship instance.

a. Attributes of different entities should not be mixed in the same relation.
b. Only foreign keys should be used to refer to other entities.
c. Entity and relationship attributes should be kept apart as much as possible.

The answer is option A) Attributes of different entities should not be mixed in the same relation.

Explanation:

A tuple is a set of attribute values in which no two distinct elements have the same name. Thus, in some accounts, a tuple is described as a function mapping names to values.

An attribute value is an attribute name paired with an element of that attribute's domain.

Informally, each tuple in a relation should represent one entity or relationship instance.

Therefore, attributes of different entities should not be mixed in the same relation.

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