When does a validation expression allow its atomic widget's field to be saved?

A validation expression is designed to enforce rules on the input provided by users into a field. The correct answer, indicating that the expression allows its atomic widget's field to be saved when it returns a null value, highlights an important aspect of how data validation operates.

When a validation expression returns a null value, it is typically interpreted as a signal that there are no errors present in the entered data. This outcome often indicates that the input meets all necessary criteria, therefore allowing the application to proceed without any hindrances in saving the data. Null in this context implies validation success, meaning there are no validation errors to prevent the user from saving.

The other options do not align with this understanding of validation logic. An error present typically signifies that the validation has failed. An empty field might trigger a validation rule that requires the field to have some data, therefore preventing saving. The action of the user clicking the save button does not determine whether the field can be saved but relies on the validation logic to check if saving should proceed at that point.