To send Opportunity values via an HTTP REST callout when a user presses a custom button in Lightning Experience:
Option B: Create a custom Visualforce quick action
A Visualforce page with a custom controller can perform the callout.
The Visualforce page can be added as a quick action on the Opportunity page.
A Lightning component can perform the callout when invoked.
The component can be added to the Opportunity page as a quick action.
Reference:
"You can use Visualforce to create custom quick actions."
- Salesforce Help: Quick Actions
Option C: Create a Lightning component quick action
"Use Lightning components to create custom actions that can be added to Lightning Experience and the Salesforce mobile app."
- Salesforce Help: Lightning Component Actions
Why Other Options Are Incorrect:
Option A:
Remote Actions are used in Visualforce for client-side calls and are not applicable here.
Performing the callout whenever the Opportunity is updated does not meet the requirement of initiating the action via a button click.
Option D:
An after-update trigger would execute on every update, not just when the user presses a button.
Using @future methods may not execute immediately and can't be invoked directly by the user action.