The correct answer is option 'B': Primary key.

A primary key is a unique identifier for a record in a database table. It uniquely identifies each record in a table and ensures that there are no duplicate values. It is used to establish relationships between different tables in a database.

A foreign key is a field in one table that refers to the primary key in another table. It creates a link between the two tables and allows for the establishment of relationships between them. The foreign key in one table is used to reference the primary key in another table.

Here's a detailed explanation of how a foreign key works:

1. Primary Key:
- A primary key is a column or a set of columns in a table that uniquely identifies each record in that table.
- It ensures that each record has a unique identifier and helps in maintaining data integrity.
- Primary keys can be composed of one or more columns, depending on the requirements of the table.
- For example, in a table of students, the primary key could be the "student_id" column.

2. Foreign Key:
- A foreign key is a column or a set of columns in a table that refers to the primary key in another table.
- It establishes a relationship between two tables by linking the foreign key in one table to the primary key in another table.
- The foreign key in one table points to the primary key in another table, creating a link between the two tables.
- For example, in a table of courses, there could be a foreign key column called "student_id" that references the primary key "student_id" in the students table.

3. Relationship between Tables:
- The relationship between two tables is established using the primary key and foreign key.
- In the example mentioned above, the primary key "student_id" in the students table is referenced by the foreign key "student_id" in the courses table.
- This relationship allows for the retrieval of data from both tables based on the shared key.
- For example, to retrieve all the courses taken by a specific student, we can use the foreign key "student_id" in the courses table to find the corresponding records in the students table.

In conclusion, a foreign key is a field in one table that references the primary key in another table. It establishes a relationship between the two tables and allows for data retrieval based on the shared key. The primary key ensures the uniqueness of each record, while the foreign key creates the link between the tables.