_______ is a constraint that can be defined only at the column level?

Answer: B Explanation: In SQL, constraints can be defined at either the column level or the table level. The NOT NULL constraint is applied directly to a column and cannot be defined at the table level. It ensures that a column does not accept NULL values. Other constraints like UNIQUE, CHECK, and PRIMARY KEY can be defined at both column and table levels. For example, a PRIMARY KEY can be defined on a single column or multiple columns together. Similarly, UNIQUE and CHECK constraints can be applied across multiple columns in a table. Therefore, NOT NULL is the only constraint that can be defined only at the column level.