the correct answer is b
d) is a complete sentence.
doctors have knowledge in anatomy, power to heal people and the nerve to proceed without hesitation. if a doctor decides to use this knowledge the wrong way, he has an obvious advantage over any other criminal. he can smartly proceed to prescribe the wrong medication to a person. also, in case of surgery, he can hurt that person to produce his or her death. sometimes, this can be done without leaving traces at all.
in "the adventure of the speckled band" doctor grimesby roylott, proceeded to kill helen's sister without leaving any trace. this prevented him from being put in prison. but since holmes is very smart, he can realize roylott moves. finally, he discovered the doctor's methods to commit such crime. this made him conclude that "when a doctor does go wrong he is the first of criminals".