A validation rule is a formula that validates the data entered by a user and prevents the record from being saved if the data does not meet certain criteria. In this case, the administrator can create a validation rule to ensure that the custom encrypted field for credit card numbers has exactly 16 digits. The other options are not applicable for encrypted fields. Reference: https://help.salesforce.com/s/articleView?id=sf.fields_about_encrypted_fields.htm&type=5 https://help.salesforce.com/s/articleView?id=sf.validation_rules.htm&type=5