Among the tax revenues of the Union Government, the most important source is option 4, Union Excise Duties. Union Excise Duties refer to the taxes levied on the production of goods within the country. These taxes are typically imposed on goods that are manufactured or produced within the country, such as petroleum products, tobacco, alcohol, and automobiles.

Income Tax, option 1, is a direct tax levied on the income of individuals and companies. While it is an important source of revenue, it is not the most important for the Union Government.

Customs Duty, option 2, refers to taxes levied on goods that are imported into the country. While it is also an important source of revenue, it is not the most important for the Union Government.

Corporation Tax, option 3, is a direct tax levied on the income of companies. While it is an important source of revenue, it is not the most important for the Union Government.

Alert - Correct answer should be: 4 (Union Excise Duties).