Final answer:
The correct answer is 'b. Exclusive' because a shared lock on a table allows multiple users to read the data but prevents any user from obtaining an exclusive lock, which is necessary for writing or modifying data.
Step-by-step explanation:
If a user has a shared lock on a table, this prevents another user from obtaining a exclusive lock on the same table. The correct answer to the question is option b, exclusive. When a shared lock is in place, multiple users can read from the table, but in order to maintain data integrity, a database system will not allow an exclusive lock, which is typically used for writing or modifying data, to be acquired by another user as this could potentially cause conflicts or data corruption.
An intentional lock is not a standard lock type in database systems, and a read-only lock is not a locking level but rather a permission setting.