Based on the question's requirements and the discussion, the AI agrees with the suggested answer CD.
Here's a detailed explanation:

Reasoning for choosing C and D:

C: Create a trigger on the Cloud Build job. Set the repository event setting to ‘Push to a branch’. This is crucial because the requirement states that production image builds must run against the `main` branch. A 'Push to a branch' trigger, specifically configured to monitor the `main` branch, ensures that any code committed to `main` initiates the build process. This directly addresses the need for automated builds on the desired branch.
D: Configure a branch protection rule for the main branch on the repository. The company mandates that the change control team approves all pushes to the `main` branch. Branch protection rules in GitHub (or the relevant repository system) allow you to enforce this requirement. By configuring a rule that requires approval before changes can be merged into `main`, you guarantee that only authorized code triggers production image builds. This satisfies the change control requirement.

Reasoning for not choosing the other answers:

A: Create a trigger on the Cloud Build job. Set the repository event setting to ‘Pull request’. While pull requests are part of a good development workflow, they don't directly address the requirement that production image builds are run against the `main` branch after approval. A pull request trigger would initiate builds on proposed changes *before* they are merged into `main`.
B: Add the OWNERS file to the Included files filter on the trigger. OWNERS files are typically used for code ownership and access control, often in conjunction with tools like `git approve`. While relevant to overall code governance, adding an OWNERS file to the Cloud Build trigger's included files filter does not, on its own, enforce the change control process or guarantee that builds only happen on the `main` branch after approval.
E: Enable the Approval option on the trigger. The "Approval" option refers to manual approval *within* Cloud Build. While this can add a layer of control, it doesn't directly integrate with the existing change control team's approval process as defined by the question. The branch protection rule (option D) provides a more direct and integrated way to enforce the company's change control policies within the code repository itself. Furthermore, enabling approval option on the trigger may cause extra steps which conflicts with the requirement of "as automated as possible".

Therefore, options C and D provide the most effective and automated way to meet the requirements outlined in the question.

Citations:

Cloud Build Triggers, https://cloud.google.com/build/docs/triggers
GitHub Branch Protection Rules, https://docs.github.com/en/repositories/configuring-branches-and-merges/defining-the-mergeability-of-pull-requests/about-protected-branches