Investigations _______ that the sudden outbreak of hepatitis was due to the consumption of contaminated water.
a) are revealing
b) have revealed
c) has revealed
would be very helpful if an explanation is also provided along with the answer.
C) is not correct as the verb is third-person singular and the subject is plural. But it could be any of the others – it all depends on what you want to say. If the investigations are revealing something, that would imply that the investigations are still going on, if the investigations have revealed something, then they revealed it in the past and/or revealed it at a time close to the present. If the investigations reveal something, then you are simply describing the investigation reavealing something.
[Not a teacher]