If the requirement is interactive, real-time access in an app UI then A fits. Canvas apps with custom connectors let you call the Web API directly from the interface. Pretty sure that's what they're looking for here, unless there's some hidden detail.
I don’t think it’s D. Canvas app (A) feels right here since it gives direct UI access to the API and custom connectors work natively. Webhook’s more for async stuff, so A is my pick unless I missed something crucial.
A is the way to go here. Canvas apps let you use custom connectors right in the UI for real-time Web API calls, which fits what the question’s after. Plug-ins and webhooks are more back-end. Unless there’s a detail I missed, A makes sense. Agree?