The correct answer is option 'D', which is the Primary key.

A primary key is a field or a combination of fields in a database table that uniquely identifies each record in the table. It is a crucial concept in relational databases as it ensures data integrity and provides a way to establish relationships between tables.

Here are some key points to understand about the primary key:

Uniqueness:
- A primary key must have a unique value for each record in the table. No two records can have the same primary key value.
- This uniqueness helps in uniquely identifying and differentiating each record in the table.

Identification:
- The primary key helps in identifying a specific record in the table.
- It acts as a unique identifier for each record and enables quick and efficient retrieval of specific data.

Data Integrity:
- The primary key constraint ensures data integrity by preventing duplicate or null values in the key field(s).
- It enforces the uniqueness of the primary key values, thereby maintaining the integrity and accuracy of the data stored in the table.

Relationships:
- The primary key of one table can be referenced as a foreign key in another table to establish relationships between the tables.
- This allows for the creation of connections and associations between related data in different tables.

Indexing:
- Primary keys are often automatically indexed by the database management system (DBMS) to improve query performance.
- The indexing of the primary key allows for faster searching, sorting, and joining of data based on the key field(s).

In summary, a primary key is a field or a combination of fields that uniquely identifies each record in a database table. It ensures data integrity, enables efficient retrieval of specific data, and establishes relationships between tables.