Introduction:
A union tax refers to a tax levied by the central government in a federal system. It is a form of taxation that is applicable across the entire country and is collected by the central government. In India, the central government is responsible for levying union taxes, while state governments have the authority to levy state taxes.

Explanation:
Among the given options, the correct answer is option 'A' - Corporation tax. Let's understand why:

1. Corporation tax:
Corporation tax is a tax imposed on the income of corporations or companies. It is levied by the central government and is applicable to all corporations operating in the country. The rate of corporation tax may vary based on the company's profits and other factors. This tax is collected by the central government and contributes to its revenue.

2. Taxes on agricultural income:
Taxes on agricultural income are not union taxes. In India, the power to levy and collect taxes on agricultural income lies with the state governments. Therefore, it is not considered a union tax.

3. Capitation taxes:
Capitation taxes are taxes levied on individuals, usually as a fixed amount per person. They are not specific to a particular type of income and are not categorized as union taxes. In India, the power to levy capitation taxes is not vested in the central government.

4. Land revenues:
Land revenues are the income generated from the ownership or use of land. They are not considered union taxes as they are primarily under the jurisdiction of state governments. The central government does not have the authority to levy land revenues.

Conclusion:
Among the given options, the only tax that qualifies as a union tax is corporation tax. It is levied by the central government on the income of corporations and contributes to the central government's revenue. Taxes on agricultural income, capitation taxes, and land revenues are not union taxes and fall under the purview of state governments.

Answer is a