Which operator is used to check the expression is not "NULL"?

Solution(By Examveda Team)

IS NULL: This operator is used to check if an expression evaluates to "NULL". It returns true if the expression is NULL, otherwise false.
NOT NULL: This operator is used to check if an expression is not "NULL". It returns true if the expression is not NULL, otherwise false. This is the operator specified in the question.
ON: The ON keyword is typically used in conjunction with JOIN operations to specify the join conditions between tables, it is not used for checking NULL values.
None of the mentioned: This option is incorrect as there is indeed an operator used to check if an expression is not "NULL", which is NOT NULL.

Therefore, the operator used to check if the expression is not "NULL" in SQL is Option B: NOT NULL.