The correct answer is C (Azure Marketplace) because Azure Marketplace is specifically designed as a digital catalog for purchasing and deploying third-party solutions, including virtual security appliances, directly to Azure subscriptions. The community discussion shows 100% consensus on this answer, with multiple comments highlighting that 'third-party' is the key keyword that points to Azure Marketplace. Azure subscriptions (A) are for managing access to services, not purchasing third-party solutions. Azure Security Center (B) is a security management tool, not a marketplace for purchasing appliances. Microsoft Store (D) is for consumer software and games, not Azure-specific virtual appliances.