The problem has two key requirements:

All applications must send data to a centralized security service.

Developers need high autonomy over firewall rules within their projects.

Prevent accidental blockage of access to the central security service.

This scenario requires a mechanism to enforce critical network policies at a higher level of the resource hierarchy while still allowing project-level flexibility.

Hierarchical Firewall Policies: Google Cloud's Hierarchical Firewall Policies (HFP) are designed precisely for this purpose. They allow administrators to define firewall rules at the organization or folder level, and these rules are inherited by all projects and VPC networks within that hierarchy. Crucially, HFP rules can be prioritized. Rules with higher priority (lower numerical value) are evaluated first. This means you can create high-priority "allow" rules for critical services that cannot be overridden or blocked by project-level firewall rules.Extract Reference: "Hierarchical firewall policies allow you to define and enforce consistent network security policies across your organization. Policies can be applied at the organization or folder level, and they are inherited by all projects and VPC networks within that hierarchy." and "Rules in a hierarchical firewall policy can take precedence over VPC network firewall rules based on priority. A rule with a lower priority value takes precedence over a rule with a higher priority value." (Google Cloud documentation: https://cloud.google.com/vpc/docs/firewall-policies-overview)

Preventing Accidental Blockage while Allowing Autonomy: By setting a high-priority "allow" rule for the central security service in a hierarchical firewall policy, you guarantee that this traffic will always be permitted, regardless of what project-level firewall rules developers might configure. This ensures the critical connectivity while still allowing developers to manage other, less critical firewall rules within their projects with high autonomy.

Let's evaluate the other options:

A. Deploy a central Secure Web Proxy and connect it to all VPC networks. Create a Secure Web Proxy policy to allow traffic to the central security service. A Secure Web Proxy is for HTTP/S outbound traffic to external web services. The central security service might not be an external web service, and this solution is focused on application-layer proxies, not general network connectivity like sending data to an internal service. Also, it doesn't directly address the challenge of developers blocking access with project-level firewall rules.

C. Create a central project to manage Shared VPC networks which will be accessible to all other projects. Administer all firewall rules centrally within this project. While Shared VPC centralizes network management, it means all firewall rules are administered centrally. This directly contradicts the requirement for developers to have "high autonomy to configure firewall rules within their projects." Shared VPC would centralize too much control for this specific scenario.

D. Use Terraform to automate the creation of the required firewall rule in all projects. Restrict rule change permissions solely to the Terraform service account. This approach automates the creation but doesn't prevent developers from creating conflicting or overriding rules in their projects (unless Terraform is used to manage all rules, again removing autonomy). It also relies on restricting IAM permissions for all firewall rules, which is against the "high autonomy" requirement for developers. Hierarchical firewall policies offer a more robust and native solution for overriding and enforcing specific rules.

Therefore, implementing a hierarchical firewall policy is the most effective solution, as it allows for the enforcement of critical security service connectivity at a higher level, while still granting developers the desired autonomy over their project-specific firewall rules.