Concept:A one-party system that is enforced through official legal provisions is called a de jure one-party system.
Explanation:The phrase “imposed by law” means the system is created and maintained through legal statutes or the constitution.
When all other parties are proscribed, they are officially banned and cannot operate legally.
This makes the ruling party the only lawful political organisation.
Such a system is based on legal authority rather than merely on social or practical circumstances.
Therefore, a legally enforced one-party system is best described as a de jure one-party system.
Answer:B. a de jure one party system