Explanation:

In the context of databases, a key is a property of the entire relation (table), rather than of the individual tuples (rows) within the relation. A key is used to uniquely identify a tuple within a relation. The correct answer to the given question is option B, "Key."

Relation:
A relation is a table in a database that consists of rows (tuples) and columns (attributes). Each tuple represents a specific instance of the entity being modeled, and each attribute represents a specific piece of information about the entity.

Key:
A key is a set of one or more attributes that uniquely identify a tuple within a relation. It ensures that each tuple is unique and allows for efficient retrieval and manipulation of data.

Property of the entire relation:
A key is a property of the entire relation because it guarantees the uniqueness of tuples within the relation. It is defined at the time of creating the relation and remains constant throughout the existence of the relation. The key defines the structure and integrity of the relation and is critical for maintaining data consistency.

Unique identification:
The key is used to uniquely identify each tuple within the relation. It ensures that no two tuples have the same combination of attribute values. This uniqueness is essential for several operations, such as searching for specific tuples, updating or deleting tuples, and avoiding data redundancy.

Individual tuples:
While each tuple within a relation is unique, the uniqueness is not a property of the individual tuples themselves. It is the result of the key constraint imposed on the relation as a whole. The key constraint ensures that the combination of attribute values in each tuple is unique, thus making the entire relation consistent and reliable.

Conclusion:
In summary, a key is a property of the entire relation, not the individual tuples within the relation. It guarantees the uniqueness of tuples and allows for efficient data retrieval and manipulation.