The correct answer is Cloud IAM. Cloud Identity and Access Management (IAM) is the GCP service dedicated to managing and securing user identities. It offers centralized control over access and permissions for GCP resources, enabling you to define detailed access policies, assign roles to users and groups, and regulate access to specific resources and actions within your GCP projects.

• Cloud Identity-Aware Proxy focuses on securing access to web applications based on user identity, not on managing identities.
• Cloud Armor is aimed at protecting applications from threats like DDoS attacks, focusing on network traffic security rather than identity management.
• Cloud KMS provides key management and encryption capabilities to secure data, but it does not specialize in managing user identities.

Thus, Cloud IAM is the correct choice for managing and securing user identities in GCP.