In Guidewire PCF Configuration, a Validation Expression is used on input widgets to enforce data quality. These expressions must evaluate to a specific return type: if the data is valid, the expression must return null. If the data is invalid, it must return a String (usually via a DisplayKey) containing the error message that will be displayed to the end-user.

To address the specific requirement of checking digits 4 through 6, developers use Gosu Slicing on the phone number string. Since Gosu (and most programming languages) uses 0-based indexing, the 4th digit is at index 3, the 5th at index 4, and the 6th at index 5. Therefore, the slice range is [3..5].

Option C is the correct implementation. It slices the NationalSubscriberNumber from index 3 to 5, converts it to a String, and checks if it equals ' 555 ' . If it matches (meaning the number is invalid), it returns the BadPhoneNumber DisplayKey. If it does not match, it returns null, signifying the data is valid.

Option A and D are incorrect because they return true on success, which is a common mistake; the UI engine interprets any non-null return value as an error message. Option B is incorrect because it uses .contains(), which would flag ' 555 ' appearing anywhere in the number, not just in the specific prefix positions requested by the business analyst. Mastering these expressions ensures that users receive immediate, accurate feedback while maintaining the integrity of the InsuranceSuite Data Model.