Rows of a relation are called a. a relation row. b. a data structure. c. an entity. d. tuples.

To answer the question "Rows of a relation are called", let’s evaluate the options provided:

1. Option A: a relation row.

   • This term is not commonly used in database terminology.

2. Option B: a data structure.

   • This is a broader term and does not specifically refer to rows in a relational database context.

3. Option C: an entity.

   • In database terminology, entities represent objects or things, but they are not specifically equivalent to rows.

4. Option D: tuples.

   • In relational databases, each row in a relation (or table) is indeed called a tuple. A tuple represents a single data record.

Conclusion

The correct answer is d. tuples. Rows in a relation are formally referred to as tuples, as they encapsulate a single set of related values corresponding to the attributes defined by the relation.