What will applying a retention date on a file ensure?

B. data will not be deleted before that date.

A retention date will ensure that a file cannot be overwritten before that date has passed. The retention date will not affect the ability to read the file. Backup copies would be expected to have a different retention date and, therefore, may well be retained after the file has been overwritten. The creation date, not the retention date, will differentiate files with the same name.