With a severe head injury (GCS 8, unequal pupils) hypotonic fluids must be avoided because they cause cerebral edema - this rules out D5W and 0.45% saline; colloids such as albumin are not used routinely. That leaves an isotonic crystalloid. 0.9% normal saline is the usual answer (bank key), but the cohort notes list both NS and Ringer's lactate as isotonic resuscitation fluids and do not say which to prefer in head injury, so B cannot be excluded from the sources alone.