To answer this question, let's go through each option to understand which constraint can be defined only at the column level:
Option A) Unique - The unique constraint can be defined at both the column level and the table level. So, this option is incorrect.
Option B) Not null - The not null constraint can only be defined at the column level. This option is correct.
Option C) Check - The check constraint can be defined at both the column level and the table level. So, this option is incorrect.
Option D) Primary key - The primary key constraint can be defined at both the column level and the table level. So, this option is incorrect.
Option E) Foreign key - The foreign key constraint can be defined at both the column level and the table level. So, this option is incorrect.
The correct answer is option B) Not null. This constraint can only be defined at the column level.