B makes more sense than any other options

B. Set up a staging database, and define external IDs to merge, clean duplicate data, and load into Salesforce.

This approach allows for the cleaning and deduplication of data before it is loaded into Salesforce, which aligns with Universal Container’s requirements. The use of a staging database provides a controlled environment to handle the data from the two legacy systems, and the definition of external IDs facilitates the merging of records. This ensures a clean, organized, and efficient data migration process.

I would say B.
You always should clean up data before importing it to Salesforce.