Honestly, I am not sure I could explain it better than the back of the book can but essentially when you do dehydrohalogenation via E2 you need to remove a a Hydrogen (hydro-) and a halogen (-halogenation) which in this case is Iodine.
Well there are two criteria for this reaction to happen: 1) Both the H- and the -I must be in the axial position 2) They also must be anti to one another
This will give you answer B. Also don't forget that bulky groups must always be in the equatorial position.
