The dependabot.yml configuration file requires a set of update configurations defined under the updates key. For each update configuration, three keys are mandatory: package-ecosystem, directory, and schedule.interval. The package-ecosystem key is essential as it specifies which package manager Dependabot should monitor for dependencies (e.g., npm, maven, pip). Without this key, Dependabot cannot identify which ecosystem to scan for outdated dependencies. The other options listed are optional parameters used for customizing Dependabot's behavior.

A. rebase-strategy: This is an optional key used to define how Dependabot handles pull request conflicts, such as by rebasing automatically.

B. commit-message: This is an optional key that allows for customization of the commit messages used in Dependabot's pull requests.

C. assignees: This is an optional key used to automatically assign specific users or teams to the pull requests created by Dependabot.

1. GitHub Docs. (n.d.). Configuration options for the dependabot.yml file. GitHub. Retrieved from https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#package-ecosystem

Reference Specifics: Under the section "Required properties for each updates entry," the documentation explicitly lists package-ecosystem as "Required" and describes its function. The same document lists rebase-strategy, commit-message, and assignees as optional parameters in their respective sections.