Which of the following is the primary principle underlying the concept of "ability to pay" in taxation?

The ability to pay principle is a foundational concept in taxation which holds that a taxpayer’s tax liability should be related to their economic capacity, i.e., their income or wealth. This principle justifies progressive taxation, where higher earners pay a larger percentage of their income in tax, ensuring fairness and equity in the tax system. Options A and E are incorrect as they do not reflect this principle. Option C is about proportional taxation which differs from ability to pay. Option D ignores differences in taxpayers’ economic situations.