The correct answer is C (Azure Marketplace) because Azure Marketplace is specifically designed for purchasing and deploying third-party solutions, including virtual security appliances, directly into Azure subscriptions. Azure Marketplace offers a curated catalog of certified third-party applications and services that can be easily deployed. Azure subscriptions (A) are the billing containers but not the purchasing platform. Microsoft Defender for Cloud (B) is a native Azure security service, not a marketplace for third-party appliances. Microsoft Store (D) is for consumer software and applications, not enterprise Azure deployments.