Answer: E

Explanation: The correct answer is E. Residency is not one of the key principles of taxation. The four main principles of taxation as outlined by Adam Smith are Equity, Certainty, Convenience, and Efficiency (Simplicity falls under efficiency or ease of administration). Residency is a concept related to tax jurisdiction and liability but is not considered a fundamental principle of taxation.

• Reasoning: Residency relates to where a taxpayer resides and is used to determine tax liability. However, it is not classified as a foundational principle guiding the design and application of tax systems. The principles focus on fairness, clarity, and ease of payment.