Assertion. There is a difference between a field being empty or storing NULL value in a field.

Reason. The NULL value is a legal way of signifying that no value exists in the field.

(a)

Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

Explanation
There is a difference between a field being empty or storing NULL value in a field. An empty field means that the field contains a value, but that value is an empty string or a space, depending on the data type. In contrast, if a column in a row has no value, then column is said to contain a NULL. Hence, the NULL value is a legal way of signifying that no value exists in the field.