Explanation/Reference: https://cloud.google.com/solutions/federating-gcp-with-active-directory-introduction

Project Billing Manager is required to manage the GCP project billing settings. We assume you can create a new billing account because you are an organization billing admin or because you are not part of an organization.

Both the roles cannot create a new billing account.
So we assume the account is already created.
Now we have to attached existing project to existing account. So it could not be B and C as they want roles to create new account which is not possible.
Both A & D are confusing because the Project Billing Manager can link & unlink a project from a billing account while a Billing Account Admin can link & unlink projects while managing user roles on the billing account. I'd go with A because option D refers to a Billing Admin and there's nothing like a Billing Admin only a Billing Account Admin.
https://cloud.google.com/billing/docs/how-to/billing-access