What defines a set of restrictions on the values allowed for the attributes of relations?

The correct answer is defined by domain constraints, which establish specific rules regarding the type of data that can be stored in a database's attributes. Each attribute in a relation is associated with a domain, which determines the acceptable values, data types, formatting, and any other limitations that may apply.

Domain constraints ensure that the values entered into the database adhere to defined standards, which can include data types (e.g., integer, string, date), ranges of values (e.g., an age must be between 0 and 120), and other criteria like uniqueness or nullability. By enforcing these constraints, domain restrictions help maintain the accuracy and reliability of the data stored in the database, preventing errors and ensuring that invalid data does not compromise the overall database integrity.

Entity integrity pertains to the uniqueness and existence of entity instances, especially regarding primary keys, while the foreign key relates to maintaining referential integrity between tables. Data integrity encompasses a broader concept that includes both the accuracy and consistency of data within the database, but it is not as specific as domain constraints in setting the restrictions on values for individual attributes. Thus, domain constraints are the most precise mechanism for defining allowable values for attributes within relations.