Final answer:
To prevent duplicates in the BookISBN column, the correct option is 'c) UNIQUE KEY', although 'd) CONSTRAINT BookISBN_UNIQUE' is also a valid approach if naming the constraint is desired.
Step-by-step explanation:
To ensure that the BookISBN column will not contain any duplicate values, you can use a UNIQUE constraint. The correct option to place in the blank space is c) UNIQUE KEY, which enforces the uniqueness of the entries in that column. However, option d) CONSTRAINT BookISBN_UNIQUE is also a valid approach when you want to name the constraint for clarity, it would be defined in the SQL statement as follows:
CONSTRAINT BookISBN_UNIQUE UNIQUE
Both options will effectively prevent duplicate values in the BookISBN column.