Firstly, alkali metals are the elements in group 1 of the periodic table.
A – Rubidium is more reactive than the hydrogen present in the water. Therefore, hydrogen would be formed and not rubidium
B – Melting points and boiling points decrease as you move down group 1. Therefore, sodium would have a higher melting and boiling point than rubidium.
C – Reactivity increases down group 1. Therefore, rubidium would react more vigorously than sodium in water.
D – All elements of the group 1 are stored in oil so they do not react with air. Therefore, rubidium is also stored in oil.
E – Rubidium has a valency of +1 and SO4 has a valency of -2. The formula would be Rb2SO4 and not RbSO4.