Based upon the valence electrons as taken from the periodic table, the formula for rubidium (rb) chloride (cl) is:

Rubidium (Rb) is in group 1, so it has a valence charge of +1.
Chlorine (Cl) is in group 17 or the halogens, so it has a valence charge of -1.

Since both elements have the same valence charges, just of opposite signs, this means that only 1 atom of each element is needed to create a stable bond. The formula for rubidium chloride will be RbCl.

Ithink the answer is a or d.