The best approach to securely connect an application in one project to an AlloyDB instance in another project, while minimizing operational overhead and following Google-recommended practices, is to use Shared VPC. Here's a breakdown:

The suggested answer is A: Provision a Shared VPC project where both the application project and the AlloyDB project are service projects.

Reasoning for choosing A:

Centralized Network Management: Shared VPC allows you to manage a common VPC network in a central host project, while allowing multiple service projects (like the application and AlloyDB projects) to participate in that network. This simplifies network administration and ensures consistent security policies.
Project Isolation: Even though the application and AlloyDB instances share the same VPC, they remain in separate projects, maintaining project-level isolation for resource management, billing, and access control.
Google Recommended Practice: Shared VPC is a recommended practice for multi-project environments in Google Cloud, as it promotes network consistency and simplifies management.
Secure Connectivity: Shared VPC enables secure private connectivity between the application and AlloyDB instances without exposing them to the public internet.
Minimizing Additional Operations: Although it involves setting up a Shared VPC, this setup is relatively straightforward. Once configured, it provides a stable and secure network foundation for inter-project communication. It minimizes the need for managing individual firewall rules or proxy configurations for each application component.

Reasons for not choosing the other answers:

B. Use AlloyDB Auth Proxy and configure the application project’s firewall to allow connections to port 5433: While AlloyDB Auth Proxy provides secure authentication, relying solely on firewall rules doesn't address the underlying network connectivity between the projects. It also adds operational overhead in managing and maintaining firewall rules. It doesn't utilize the recommended practice of Shared VPC for cross-project networking.
C. Provision a service account from the AlloyDB project. Use this service account’s JSON key file as the --credentials-file to connect to the AlloyDB instance: While service accounts are essential for authentication and authorization, using a JSON key file directly introduces security risks. It does not address the core problem of network connectivity between projects and is not a recommended security practice. It is generally recommended to use workload identity instead of service account keys.
D. Ask the database team to provision AlloyDB databases in the same project and network as the application: This approach eliminates project isolation, which is a key requirement specified in the question. It also violates the principle of separation of concerns, where the database and application should be managed independently. Moving databases may also involve significant migration efforts and potential downtime.

Therefore, Shared VPC provides the most secure, manageable, and Google-recommended solution for connecting your application to the AlloyDB instance across different projects.

Shared VPC Documentation, https://cloud.google.com/vpc/docs/shared-vpc