Answer:
A nanobot can do all of these things depending upon the purpose and way it is structured. It also depend on mind of maker and their potential influence
For example a humanatarian will make it for option D because he want to save life not waste them
But for the evil minded the option a will be a right choice
A medical operator will go with c or d
Extracting electrons is odd and not the real cause a nanobt will be used for but still it can do this as well but it is least likely
So it is difficult to determine correct option it depend on context which is not apparent
I hope it helps