The Answer: Rijndael. Rijndael is the new approved method of encrypting sensitive but unclassified information for the U.S. government. It has been accepted by and is also widely used in the public arena as well. It has low memory requirements and has been constructed to easily defend against timing attacks.
The following answers are incorrect: Twofish. Twofish was among the final candidates chosen for AES, but was not selected.
Serpent. Serpent was among the final candidates chosen for AES, but was not selected. RC6. RC6 was among the final candidates chosen for AES, but was not selected.
The following reference(s) were/was used to create this question:
ISC2 OIG, 2007 p. 622, 629-630 Shon Harris AIO, v.3 p 247-250