The ratio of volumes occupied by $1$ mole of $O_2$ and $1$ mole $C{O}_2$ under identical conditions of temperature and pressure is

The correct option is C $1:1$
According to Avogadro's Law two dissimilar ideal gases occupying the same volume at a given (constant) temperature and pressure must contain an equal number of molecules.

Hence, the ratio of volumes occupied by the given two gases which have equal number of molecules will be 1:1 at same temperature & pressure.