I think B is the correct answer

I believe B is the correct one.

A and C - no because - "as quickly as possible after the deal is closed"
D - no because - outbound messages support only one object - "all opportunity information, including products and billing contacts"

D: Outbound message

My second thought is B with async sending in Ququable or Batch. In question data from 3 objects must be sent "opportunity information, including products and billing contacts" but in outbound message you can choose only one object. Outbound is also configured not written in code.

D should be the correct answer

I would say D. If only an external system is able to consume it.

How come it is A?

Maybe A?
Callouts must be made asynchronously from a trigger so that the trigger process isn’t blocked while waiting for the external service's response. The asynchronous callout is made in a background process, and the response is received when the external service returns it. To make an asynchronous callout, use asynchronous Apex such as a future method. See Invoking Callouts Using Apex for more information.
https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_triggers.htm