When users need to see multiple error messages at a time related to their input on a form, client-side validation is the preferred method. This allows immediate feedback on the input fields without the need to submit the form to the server.
Option B (Client-side validation) is correct because it enables validation of multiple fields before submitting the form, allowing users to see all error messages at once for the fields that fail validation.
Options A (Apex REST), C (Custom validation rules), and D (Next Best Action) are not directly related to the validation of form inputs in the context of a Lightning web component.
References:
Lightning Web Components Developer Guide: Lightning Web Components and Salesforce Data