Probably the reason in the above sentence is that "Signaling" is not a countable noun and hence cannot be made plural by adding "s". So the last option is taken out. The problem now is between the first two; is it OK to use "were" with uncountable nouns such as this (as here "signaling" actually refers to both "biochemical signaling" and "hormonal signaling") or should it be strictly "was"?