To be honest there is no answer for that because a thermostat is a measuring device to measure temperature I hope this helps you understand why there is no answer
A synonym for thermostat could be a term that has to do with measuring energy. But an antonym for thermostat would probably be something that measures the cold. But you can't measure the absence of energy so there might be no antonym I suppose.
Number 3 would be the only logical choice in my opinion because everything else would be in a narrative or story of some sort. Expository essays are exclusively informational.
Some sources will state historic events, however if they were written a long time AFTER the events happen it is not credible because the person who wrote the source was not present at the time of the event.
