Final answer:
If a user violates a validation rule by entering a value such as "100" in a field with a "< 50" rule in Access, the error message shown is specified in the Validation Text property of the field.
Step-by-step explanation:
If the Quantity field has "< 50" as its validation rule, and a user tries to enter "100" in the field, Access displays the message in the Validation Text property. This property is used to define the error message that the database user will see if they attempt to enter data that violates the validation rule set for a particular field. In Microsoft Access, validation rules are used to restrict the data that can be entered into a field, ensuring data integrity and consistency.