Which ordering of compounds is according to the decreasing order of the oxidation state of nitrogen?

For one mole of a van der Waals gas when b = 0 and T = 300 K, the PV vs. 1/V plot is shown below. The value of the van der Waals constant a (atm L² mol^$$-$$2) is

The number of aldol reactions that occur in the given transformation are

The colour of light absorbed by an aqueous solution of CuSO₄ is