Final answer:
To mark the Category column as unused and verify its unavailability in the BOOK_PRICING table, you use SQL commands specific to the database system, such as ALTER TABLE in Oracle. The column then becomes unavailable for use and can be confirmed through data dictionary views.
Step-by-step explanation:
The question pertains to marking a column as unused in a database table and verifying that it is no longer available. To mark the Category column of the BOOK_PRICING table as unused, you typically use a database-specific SQL command. For example, in Oracle, you can use the ALTER TABLE BOOK_PRICING SET UNUSED (Category) command.
After the column is marked as unused, it is not immediately dropped from the table; instead, it becomes unavailable for use in SQL queries. To verify that the Category column is no longer available, you may query the data dictionary views. For example, in Oracle, you might query the ALL_TAB_COLUMNS view to confirm that the Category column is no longer listed for the BOOK_PRICING table.