The correct answer is B. Set field-level error message to display on the page. The study guide presents this scenario and identifies B as the answer. The validation rule already determines whether the phone number satisfies the required formatting and length conditions. From a UX perspective, the next objective is to communicate the validation failure at the location where the user must correct it. A field-level error message places the feedback adjacent to the phone field, reducing the user's effort to determine what went wrong and where corrective action is required.
Option A, In-App Guidance, is better suited to onboarding, contextual instruction, feature adoption, or process coaching. It is unnecessarily indirect for a deterministic input-validation error.
Option C, making the field required, only ensures that some value is entered. A required-field setting does not validate whether the phone number contains the correct number of digits or follows the required formatting pattern.
The UX principle being tested is effective error prevention and recovery. When a user enters invalid information, the interface should provide specific, contextual, and actionable feedback instead of forcing the user to search for the affected field.
Study Guide reference/topics: Validation Rules, field-level errors, error prevention, form usability, Declarative Design.
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