Concept:An unwritten constitution is not contained in a single formal document; it is based on laws, conventions, and traditions.
Explanation:The United Kingdom has no single constitutional document.
Its constitution is drawn from statutes, common law, conventions, and authoritative texts.
Other countries listed, like the USA and Nigeria, have written constitutions as supreme laws.
The Soviet Union also operated under a written constitution.
Therefore, the UK is the best example of a largely unwritten constitution.
Answer:D. United Kingdom