Chemoreceptors select one: a. are excited by odorant molecules. b. can cause strong behavioral responses. c. do not experience fatigue. d. both a and b e. all of the above

I think the answer should be
d. both a and b

There are a few chemoreceptors in the body but this question seems to ask for the olfactory receptor. Olfactory receptor could detect the presence of a volatile chemical that you can call as an odorant. It also linked to maternal behavior. 
But olfactory receptor can become "fatigue" because it will adapt to smell that linger too long. That is why most people with bad odor might not notice it if no one doesn't point it out.