According to Google Cloud Hybrid Connectivity documentation, Cloud VPN is the standard and recommended solution for establishing a secure, encrypted connection over the public internet between an on-premises network and a Google Cloud VPC. This aligns with Cymbal’s requirement for "secure communication" while maintaining manageability. Unlike SSH tunnels (Option B), which are fragile and difficult to scale at an enterprise level, Cloud VPN utilizes IPsec protocols to create a stable and encrypted site-to-site tunnel.

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To satisfy the "secure and manageable" criteria, the implementation must include strictly defined VPC Firewall Rules. This ensures the principle of least privilege is applied, restricting on-premises systems to only the necessary cloud resources and ports required for their specific business functions. VPC Peering (Option D) is technically incorrect here as it is designed for connecting two VPCs within the cloud, not for on-premises to cloud connectivity. A Bastion Host (Option A) provides a secure gateway for administrative login (SSH/RDP) but does not provide the transparent, network-level connectivity required for the automated "legacy file-based integrations" and "data transfers" mentioned in Cymbal’s environment. By using Cloud VPN with granular firewalling, Cymbal achieves a secure extension of their data center into Google Cloud.