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A primary key in a database is used to uniquely define a row. It is a unique identifier for a record in a table. No two rows in a table can have the same primary key value. It cannot contain null values.

Here are the steps to understand it:

1. A primary key is a field in a table which uniquely identifies each row/record in a database table.

2. Primary keys must contain unique values. A primary key column cannot have NULL values.

3. A table can have only one primary key, which may consist of single or multiple fields.

4. When multiple fields are used as a primary key, they are called a composite key.

5. If a table has a primary key defined on any field(s), then you cannot have two records having the same value of that field(s).

So, the correct answer is "A Primary key will uniquely define a row".