The correct option is B Fatal
'A disease which ends in death' is option C, Fatal. Option A, 'Contagious' is a disease that can spread from one to another by touch. Option B, 'Lethal' means very harmful or destructive. Option D, 'Deadly' means something that could cause irreparable damage. Thus, options A, B and D are incorrect and the correct word the given phrase is option C, Fatal.