Answer
Choice (a) MgCl is the correct answer choice.
Work Step by Step
The question is asking for the incorrect chemical formula of the compounds given. All answer choices contain the Cl$^{-}$ ion which means that we only have to look at the metal ions.
Choice (b) gives the correct formula because a Ca$^{2+}$ ion with a Cl$^{-}$ ion yields CaCl$_{2}$
Choice (c) gives the correct formula because a Al$^{3+}$ ion with a Cl$^{-}$ ion yields AlCl$_{3}$
Choice (d) gives the correct formula because a Rb$^{+}$ ion with a Cl$^{-}$ ion yields RbCl
Therefore Choice (a) is incorrect because a Mg$^{2+}$ ion with a Cl$^{-}$ ion yields MgCl$_{2}$ not MgCl
