The correct answer is 'Overwriting the drives multiple times with different patterns,' which is a data sanitization process that ensures all previously stored data is unrecoverable by writing patterns of zeroes, ones, or random data onto the hard drive several times. Simple deletion is inadequate as data recovery tools can restore deleted files. Standard formatting is likewise insufficient since it often leaves data recoverable. While 'Encrypting the data and discarding the keys' may seem secure, it is not a data sanitization method because without physical destruction or overwriting the encrypted data can potentially be decrypted if the encryption is broken in the future.