The correct answer is A, AWS Trusted Advisor. This service provides a series of checks that analyze your AWS environment and offer recommendations to optimize costs, improve security and performance, and follow best practices. It helps users to maintain service limits, reduce costs by identifying idle resources, and enhance performance by suggesting best practices for AWS services. The other options, such as AWS Service Catalog, AWS Ground Station, and Amazon GuardDuty, serve different purposes and do not provide cost optimization and best practice recommendations.