In a relational database table, which type of key is specifically used to uniquely identify each individual row (record) in that table?

Introduction / Context:
In relational database design, every table that stores real world entities needs a reliable way to distinguish one row from another. Without a unique identifier, tasks such as updating, deleting, or referencing a specific record become error prone. This question tests understanding of the concept of keys in a relational table and asks which key truly guarantees that each row is uniquely identifiable.

Given Data / Assumptions:

• We are working with a relational database table that stores multiple rows of data.
• We need a key that uniquely identifies each individual row.
• The options mention different types of database objects and constraints such as primary key, foreign key, index, non key attribute, and check constraint.

Concept / Approach:
In relational databases, a primary key is a column or a minimal set of columns that uniquely identify each row in a table. A primary key enforces two main properties: uniqueness (no two rows share the same primary key value) and non null (every row must have a value for the primary key). Other objects like foreign keys, non unique indexes, and check constraints play important roles, but they are not designed specifically to guarantee unique row identification in the same way as a primary key.

Step-by-Step Solution:
Step 1: Recall that a primary key is defined so that each value appears only once in the table and cannot be null. Step 2: Recognize that this property of uniqueness plus non null values allows the primary key to act as the unique identifier for each record. Step 3: Compare this with a foreign key, which is used mainly to enforce referential integrity by pointing to a primary key or candidate key in another table. Step 4: Note that non unique indexes can speed up searches but do not enforce uniqueness, and non key attributes are ordinary columns without any uniqueness guarantee. Step 5: Understand that a check constraint validates that data satisfies a logical condition but does not necessarily guarantee that rows are unique. Step 6: Conclude that the only option whose core purpose is to uniquely identify each row in a table is the primary key.

Verification / Alternative check:
A simple way to verify the answer is to imagine operations such as updating one specific student in a student table. The application would typically use the student primary key value to locate that exact row. Foreign keys may repeat many times, non unique indexes allow duplicates, and check constraints focus on rules such as value ranges. Therefore, only the primary key satisfies the requirement that each row has one and only one unique value.

Why Other Options Are Wrong:
Foreign key that links to another table is used to enforce relationships between tables and may contain duplicate values. Non unique index created only for faster searching improves performance but does not stop duplicate values. Non key attribute that stores descriptive information is just a regular column and can easily repeat. Check constraint defined to validate column values enforces conditions such as ranges or patterns but does not ensure that each row is unique.

Common Pitfalls:
Learners sometimes think that any unique looking column or any index can serve as a unique identifier. Another common confusion is between primary keys and foreign keys. While both involve keys, the primary key identifies rows within its own table, whereas a foreign key references keys in another table. It is also important to remember that a table can have only one primary key constraint, although that primary key may consist of multiple columns.

Final Answer:
The correct key that uniquely identifies each row in a relational table is the Primary key.