C. The lowest mass will be the element in period 1, group 1 (this element is hydrogen)
That looks like cells of a multicellular organism, so B.
The symbol for the hydronium ion is H3O+.
An hydronium ion is usually formed when an acid is present in water. The hydronium ion is made up of three atoms of hydrogen and one atom of oxygen, thus an hydronium ion is a water molecule which has gained an extra positive hydrogen ion.
I do not have my reference table out to see the periodic table but in order to achieve the answer pull out the periodic table. You now know that this element is going to be a metal because it is a good conductor of electric and is malleable. Next you will find both cesium and radon and look in between them to see which one is a metal that has an atomic number that lies between both radon and cesium
I don't understand . Sorry :(
