The correct answer is C. Equity of tax.
Normative economics is a branch of economics that deals with the study of how the economy should work, rather than how it actually does work. It is concerned with making judgments about economic policy and with developing theories about how the economy should be organized.
Equity of tax is a normative concept that refers to the fairness of the tax system. It is concerned with whether the tax system is fair to taxpayers, and whether it distributes the tax burden in a way that is considered to be just.
Incidence of tax is a positive concept that refers to the economic impact of a tax. It is concerned with who bears the burden of a tax, and whether the tax is paid by the person who ultimately bears the economic cost of the tax.
Effect of tax on the capacity willingness to work is a positive concept that refers to the impact of a tax on the incentive to work. It is concerned with whether a tax reduces the incentive to work, and whether it leads to a decrease in the supply of labor.
Therefore, the only option that is related to normative economics is C. Equity of tax.