Final answer:
In Microsoft Access, when a table is defined with a primary key, you are required to enter a unique, non-null value for that key during data entry to maintain data integrity.
Step-by-step explanation:
When a table is defined with a primary key in Microsoft Access, the database system enforces a rule that requires a unique, non-null value to be entered for the primary key field every time a new record is added. This process is known as data entry. A primary key uniquely identifies each record in a table, and this constraint helps maintain data integrity by ensuring that there are no duplicate records and that no record exists without an identifiable key.