TheONkeyword specifies the field used by anindexwhen creating it in SQL.

Example Usage:

sql

CREATE INDEX idx_employee_name

ON Employees(Name);

Here, an indexidx_employee_nameis createdon the Name column.

Thisimproves query performancewhen filtering by Name.

Why Other Options Are Incorrect:

Option B (IN) (Incorrect):Used in queries to match values in a set, not for indexing.

Option C (UNIQUE) (Incorrect):Ensures a column has unique values but doesnot define an index field.

Option D (CHECK) (Incorrect):Used forvalidating column values, not for indexing.

Thus, the correct answer isON, as it defines thecolumn on which an index is created.