The natures of the oxides $CrO$ and ${\mathrm{CrO}}_3$
respectively,

The oxides of metal having their lower oxidation state are basic and having higher oxidation state are acidic or amphoteric. Here, in $\mathrm{CrO}$ the oxidation state of $\mathrm{Cr}$ is +2 and in ${\mathrm{CrO}}_3$ the oxidation state of $\mathrm{Cr}$ is +6. Hence, the correct answer is 4) basic and acidic.