Which of the following is required in order for BitLocker to be enabled?

BitLocker can be used without TPM via group policy changes allowing USB drives for the encryption key.

Via http://windows.microsoft.com/en-us/windows-vista/bitlocker-drive-encryption-overview:
“BitLocker can also be used without a TPM. To use BitLocker on a computer without a TPM, you must change the default behavior of the BitLocker setup wizard by using Group Policy, or configure BitLocker by using a script. When BitLocker is used without a TPM, the required encryption keys are stored on a USB flash drive that must be presented to unlock the data stored on a volume.”