For a gas R/Cv . This gas is made up of molecules which are

The correct option is Monoatomic.

Let us understand the relationship between the gas constant $R$, the molar heat capacity at constant volume $C_v$, and the degrees of freedom of a gas molecule.

From the kinetic theory of gases, the molar heat capacity at constant volume $C_v$ for a gas having $f$ degrees of freedom is given by the formula:
$C_v=\frac{f}{2}R$
where $R$ is the universal gas constant.

Rearranging this formula to find the ratio $\frac{R}{C_v}$ gives:
$\frac{R}{C_v}=\frac{2}{f}$

According to the question, we are given that:
$\frac{R}{C_v}=0.67$ (which is approximately $\frac{2}{3}$).

Comparing the two expressions:
$\frac{2}{f}=\frac{2}{3}$
This gives the number of degrees of freedom as:
$f=3$

A gas with $3$ translational degrees of freedom (and no rotational or vibrational degrees of freedom at standard temperatures) consists of single, independent atoms. Therefore, the gas is made up of monoatomic molecules.