The correct answer to this question is D, Validation Rule. Before a record can be saved by a user, these rules can be put in place to verify that the data entered by the user meets the administrators standards. Validation rules can be either formulas or expressions. These formulas and expressions are used to evaulate the data entered, resulting in a True or False response.
An example of a validation rule can deal with an address zip code. This rule could be used to ensure the address is in the correct format. If it is not, an error message such as "United States Postal Code must be in five digit format" can appear.