Answer:
is an infectious disease ...
True,but I think a lot of health disorders are not noticeable
it could also just be someone who is a witness to the other two
What excerpt there is nothing except the question!
I'd have to say d. the hand, because every human (for the most part, no offence intended to anyone) has a hand and is able to make good (or bad, rather) use of it. It's what a person always has access to at any given time or place, even if they don't have access to one of the other options.