What kind of Azure resource should you provision to securely store subscription keys for a Cognitive Services resource?

Provisioning Azure Key Vault is the most appropriate solution for securely storing subscription keys for a Cognitive Services resource. Azure Key Vault provides a centralized way to manage secrets, encryption keys, and certificates, offering a high level of security. It incorporates features such as access control and auditing, which are essential for managing sensitive information like API keys and connection strings.

Using Azure Key Vault not only allows for secure storage but also supports policies for managing access to these secrets, ensuring that only authorized applications and users can retrieve the keys. Additionally, it helps in compliance and regulatory requirements by offering a secure environment for sensitive data.

In contrast, while Azure Storage and Azure Blob Storage could technically store the keys, they do not offer the same level of security and management features as Key Vault. Azure App Service is primarily designed for hosting web applications and services, not for securely storing sensitive configuration values. Therefore, Azure Key Vault stands out as the best option for securely managing subscription keys.