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