First question is to do with reactivity series - the higher the element is in the series the greater the tendency to oxidise. From greatest to least tendency in your list: sodium, calcium, aluminium, zinc, and iron.
(a) cannot be deduced: hydrogen peroxide is liquid. If both were gases it would be true. (b) is untrue, according to my calculations 4g of oxygen should be produced.
It should be as centrally in the liquid as possible - not touching the glass. When a liquid begins to boil its temperature does not increase, all energy is spent breaking bonds - so entire liquid should be same temperature for a while.
Oldak.
PS glance. Surely above the liquid you'd be measuring the temperature of both the air and the gas? The gas can also, of course, exceed the boiling point.