The correct answer is A, which refers to the AWS Support concierge service. This service is included with the AWS Business Support plan and provides assistance with billing and account-related inquiries. Options B, C, and D are not included by default in the Business Support plan. The AWS DDoS Response Team (B) is part of AWS Shield Advanced, a separate service. The AWS technical account manager (C) is available for an additional fee and is not included by default. The AWS Health API (D) is a publicly available service and is not exclusive to any specific support plan. Therefore, the only service or feature that is available by default with an AWS Business Support plan is the AWS Support concierge service.