The correct answer is D, AWS Marketplace. AWS Marketplace is an online store that offers a wide selection of third-party software, including security solutions, that can be used on AWS. It provides a platform for customers to find, compare, and purchase software solutions that meet their specific needs. Vendors can list their software offerings on the AWS Marketplace, making it a convenient and centralized location for customers to discover and acquire the software they require. The other options are not the correct place to purchase the security software as a service on AWS. AWS Partner Solutions Finder helps you find AWS Partner solutions, AWS Support Center is for getting support from AWS, and AWS Management Console is for managing AWS resources, not for purchasing third-party software.