Union Tax:
Union tax refers to the taxes that are levied and collected by the central government of a country. These taxes are applicable across the entire country and are used to fund various government initiatives and services. In the context of India, union taxes are levied and collected by the central government of India.

Options:
Let's analyze each option to determine which one qualifies as a union tax:

a) Corporation tax:
Corporation tax is a direct tax imposed on the income of companies and corporations. It is levied by the central government and is applicable to all companies operating in India, regardless of their location. Therefore, corporation tax qualifies as a union tax.

b) Taxes on agricultural income:
Taxes on agricultural income are not levied by the central government. In India, the power to tax agricultural income lies with the state governments. Therefore, taxes on agricultural income do not qualify as a union tax.

c) Capitation taxes:
Capitation taxes are taxes that are levied per person, regardless of their income or wealth. In India, the power to impose capitation taxes lies with the state governments. Therefore, capitation taxes do not qualify as a union tax.

d) Land revenues:
Land revenues refer to the income generated from the use or sale of land. In India, the power to collect land revenues lies with the state governments. Therefore, land revenues do not qualify as a union tax.

Conclusion:
Among the given options, only corporation tax qualifies as a union tax. It is a direct tax imposed on the income of companies and corporations by the central government. The other options, such as taxes on agricultural income, capitation taxes, and land revenues, are either levied by the state governments or do not fall under the category of union taxes.