The correct option is Use a single shared repository and open pull requests from feature branches.

This shared repository with pull requests from feature branches keeps all work in one place and supports a clean review process. Each engineer works on a feature branch and opens a pull request so teammates can review changes and automated checks can run before merge. This helps a small team iterate daily while keeping history organized and the default branch stable.

Branch protection and required reviews work best with this feature branch pull request model. It simplifies permissions and reduces coordination overhead while preserving a clear source of truth.

Have each developer fork the repository and submit pull requests across repositories spreads changes across separate repositories and introduces extra steps to keep forks synchronized. That model is better for external contributors rather than a small team that wants centralized collaboration.

Create a separate Git submodule for each contributor misapplies submodules since they are intended for including independent projects as dependencies. It increases complexity and does not improve collaboration or reviews within one repository.

Commit straight to the default branch without creating branches bypasses peer review and automated checks and increases the risk of breaking the main line. It does not meet the requirement for a straightforward workflow that supports code review.

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