Under what conditions is a gas closest to properties of an ideal gas? Why?
Under conditions of high temperature, and low pressure.
The Ideal Gas Law is an equation of state, that ignores (i) the size of the molecules, and the actual molecular volume; and (ii) interactions between the molecules. (We could add a third condition to the given list: the gas is mono-atomic, and of course this applies to only the inert gases.)
The van der Waals equation of state tries to account for these deviations from ideality by including terms that represent molecular size, and intermolecular forces.