Question

_________ is the systems reaction to the stimulus.

a. Response
b. Irritability
c. Adaptavility

Answer 1

I think it's A(response) but i'm not certain. 

Answer 2

A. Response is the system's reaction to the stimulus.
It doesn't have to be irritability - not every stimulus is unpleasant. And also, it's not adaptability, because you cannot always adapt to a stimulus, for example, to pain.