The correct answers for Salesforce Release Management best practices are:

A. Publish a release calendar for each phase of the release.
A release calendar ensures all stakeholders are aware of timelines for development, testing, and deployment.
It helps in aligning resources and managing expectations across teams.
B. Regression testing is mandatory for each release.
Regression testing ensures that new changes do not inadvertently break existing functionality.
It is critical for maintaining the stability of the Salesforce org.
C. Design the right sandbox strategy for the release.
A well-planned sandbox strategy allows for efficient development, testing, and staging of changes before deploying them to production.
It ensures that the appropriate environments are used for development, QA, and UAT.
Why Not D or E?
D. Release sign-off is only required for Production.

Sign-offs are required at multiple stages (e.g., testing, UAT) to ensure the quality of the release. Limiting it to just production is not a best practice.
E. Maintain a pre/post deployment checklist for each release.

While useful, this is more of an operational detail than a fundamental best practice. The focus should be on overarching processes like regression testing, calendar planning, and sandbox strategies.
Final Answer:
A, B, C....