Find the total number of metals which make a thin protective layer of its oxide on treatment with conc.${\mathrm{HNO}}_3$

Due to the thin and inert protective coating of aluminum oxide that is created on the metallic surface as a result of the interaction between aluminum metal and airborne oxygen, aluminum metal is not damaged by nitric acid in any concentration.

Because of the development of the passive film, or oxide layer, on the metal's surface, chromium is essentially the metal that will not dissolve in the concentrated nitric acid (${\mathrm{HNO}}_3$).

The metal will thus be unable to respond or demonstrate the action in the reaction due to the creation of the oxide layer on the surface.

So, Al and Cr is correct option.