Which of the following is NOT an explicit data typ

Which of the following is NOT an explicit data type reference?

SQL NULL is a term used to represent an absence of a value. It is used to mark empty fields in a table.
NULL is not a data type itself, it just marks the absence of a value.
All the other choices represent valid data types.

Theory

sql - defaults and null values
NULL values are 'nothing' values. When a value is null, it means the value is empty and contains no value -- not even '0'. NULLs are unique data types that are usually the default setting for all table columns.
The SQL developer may specify to allow or disallow the NULL values eliminating the possibility of running across 'empty' table columns when creating a SQL table. If the developer chooses not to allow NULL values he/she may specify a custom default value instead of the NULL (nothing) value. Primary Key table columns do not allow NULL values since this column's sole purpose is to be the unique identifier for a table column.
By default, NULL values are allowed on all newly created table columns meaning a table column is allowed to be 'empty', except primary key columns.
Read more: sql - data types