The answer is E, less than one percent.  The total number of head injury fatalities in the US is around 75,000 per year.[i]  (Estimates vary, up to 115,000).  The total number of bicycling fatalities, from all injuries, is only about 800.  The number of cyclists killed annually due to head injury is a maximum of 600, probably much less.  That's not merely low; That's less than 1%!

[i].  Data on fatalities from "Epidemiology of Head Injury," J.F. Kraus, in Head Injury, Cooper, P.R. ed., William & Wilkins, 1993.  Cyclist fatality data from NHTSA, www.nhtsa.dot.gov/.