Correct answer should be C. Queueable

We need to make a callout to a RESTful web service for each Account as it is saved, and the service can handle only one record per request.

🔹 Here’s why Queueable Apex is the best fit:
Supports callouts (@future also supports this, but... keep reading)
Flexible – You can pass complex objects like a single Account
Chaining is allowed, so you can handle one record per call if needed
More readable and manageable than @future methods
Better transaction control and error handling

good question, I think A is not entirely correct
The process builder itself does not support callouts, after all, you need to build, for example, an invocable method that can make a callout and most likely you'd have to use future method to decouple transactions (callout after dml won't work in the same transaction)

On the other hand, C and D require some point that will start them (e.g. a trigger),

B - definitely out, even though the workflow can make an outbound message, they are soap-based

I would consider any option between C and D.
Any thoughts guys?