Set Primary key
Step 1: Open a table in Design view.
Step 2: Select the field or fields you want to define as the primary key or click the row selector for the desired field.
Step 3: Click Primary Key on the toolbar.
After clicking on primary key, selected column will be look as below image
Note: - When you save a new table without setting a primary key, Access prompts you to create one. If you choose Yes, Access creates an ID field that uses the AutoNumber data type to provide a unique value for each record. If your table already includes an AutoNumber field, Access uses that field as the primary key.