Out of the given options, the equation used to determine the density of a gas is:

Perfect gas equation

Explanation:

The Arrhenius equation relates the rate constant of a chemical reaction to temperature.
The Einstein equation describes the relationship between mass, energy, and the speed of light.
The Faraday equation relates the amount of electric current to the amount of chemical change.
Perfect gas equation (PV = nRT) relates pressure (P), volume (V), number of moles (n), gas constant (R), and temperature (T). By rearranging the equation, you can derive the ideal gas density formula:

density (ρ) = n/V = (P / RT)

Therefore, the perfect gas equation allows you to calculate the density of a gas based on its pressure, temperature, and the universal gas constant