Among the choices, the polyatomic ion is only the third choice, NH4+ where it is composed of nitrogen one atom of nitrogen and 4 atoms of hydrogen. Choice 2 is a monoatomic atom being composed only of calcium. Then, the first and fourth choices are not ions because they are not charged.
My educated guess should be the 3rd one
It is a physical change because you can not put it back like it was
CO2 ; H20- They are the only ones that, on both sides, combined with another element and bonding of atoms
The reaction is actually endothermic because delta H is positive, indicating that it absorbing heat.
