The correct answer to this question is incorrect.

The keyword "NULL" represents the special value NULL.

In the context of (relational database) boolean logic, there are three possible values: TRUE, FALSE and NULL. In a boolean context, NULL means NULL. The predicate (e.g.)

WHERE 1=NULL

evaluates to NULL. This is not unknown, it is very specifically known to be the special value NULL. When combined with other boolean expressions with AND and OR operators, the value of NULL is well defined, it's not "unknown".

Yes, NULL CAN be used represent the idea "the value for this expression is unknown". But it certainly is not the only meaning. A NULL value can be used to represent other concepts, such as "a value is not applicable" for a particular row, or a value is "not yet assigned", or that some event has not yet occurred. These meanings are distinct from the concept of UNKNOWN. (We can use a NULL to represent something that is known.

The assertion that NULL means UNKNOWN is incorrect. That notion is inadequate and incomplete.