State the key that can be used as a primary key.

The key that can be used as a primary key is the one that uniquely identifies each record in a database table. Here are the steps to identify it:

1. Uniqueness: The primary key must contain a unique value for each row of data. It cannot contain null values.

2. Stability: The primary key values should not change over time. Once a primary key is defined and data is added to the table, you should avoid changing the primary key value.

3. Simplicity: Ideally, the primary key should be as simple as possible, preferably with a single column. However, it's not uncommon to have primary keys that are based on multiple columns, known as composite keys.

4. Sequential: It's often beneficial for the primary key to be sequential, such as an auto-incrementing integer. This is not a strict requirement, but it can improve performance for large tables.

Examples of primary keys could be a Social Security Number in a table with personal details, a product code in a products table, or an order ID in an orders table.