To communicate a property change up a containment hierarchy in Lightning Web Components (LWC), the child component should dispatch a custom event with the detail of the change. The parent component listens for this event and handles it accordingly.
Option A is the correct method because it creates a new custom event with the detail property containing the new value of passthrough. This event is then dispatched, and the parent component can listen for this event to handle the updated value.
Options B, C, and D are incorrect because they either don't pass any data with the event or they don't use the detail object which is the standard way to pass data with custom events in LWC.
References:
Lightning Web Components Developer Guide - Communicating with Events