In a scenario where an integration requires making a callout to an external web application, Apex is often used to perform callouts. The callout must be asynchronous when being made from a trigger because a callout cannot be made from a synchronous operation like a trigger due to the possibility of long-running transactions.
Option A (An Apex trigger that calls an @future method that allows callouts) is correct because it enables the trigger to perform an asynchronous callout to the external web application, which is necessary for sending JSON payloads and receiving the resized images.
Options B, C, and D do not fit the requirement as well as option A because platform events and flows are typically not used for making direct callouts to external systems in the context described.
References:
Apex Developer Guide: Apex Web Services and Callouts