What is the premise that supports "Hot water sits in a heating tank?"
Stagnant water loses oxygen supports the idea that hot water that sits in a heating tank loses oxygen, which leads to the conclusion that cold water is better than hot water for cooking.
The question was not about the function of the statement "Hot water that sits in a heating tank loses oxygen." The question concerned the statement "Hot water sits in a heating tank." There is no premise in the stimulus that supports that...it wouldn't even be necessary.
Did the question asked what premise did that phrase support or what the overall phrase's function is? Because I think the phrase definitely supported a premise of the overall argument - the next sentence which said, water that sits longer loses the most oxygen. There were at least two premises, and then the subsidiary conclusion, water with more oxygen tastes better (or something like that) and then the main conclusion.
Actually water that sits longer would support the idea that hot water sits in a heating tank all day has less oxygen, which directly supports the main conclusion. In any case, I'm giving up on this one. We'll never know the answer, which is kind of sad because I really would have liked to see who was right about this one. I have accounting homework to do. Although, truthfully, arguing about this is much more fun.