The next clue informs us that the data.txt file contains the password enocded in base 64. We merely have to cat the file contents and pipe the results to the base64 command, using the -d switch to decode the string.
bandit10@bandit:~$ cat data.txt | base64 -d
The password is XXXXXXXXXXXXXXX (actual password is not shown)
bandit10@bandit:~$