The best assurance of data integrity after file transfers is hash values. Hash values are unique strings that are generated by applying a mathematical function to the data. Hash values can be used to verify that the data has not been altered or corrupted during the transfer, as any change in the data would result in a different hash value. By comparing the hash values of the source and destination files, one can confirm that the data is identical and intact.

The other options are not as effective as hash values for ensuring data integrity after file transfers. Check digits are digits added to a number to detect errors in data entry or transmission, but they are not reliable for detecting intentional or complex modifications of the data. Monetary unit sampling is a statistical sampling technique used for auditing financial statements, but it is not applicable for verifying data integrity after file transfers. Reasonableness check is a validation method that checks whether the data falls within an expected range or format, but it does not guarantee that the data is accurate or consistent with the source.