Concept:A unitary state is a country where a single central government holds supreme power and makes laws for the whole country.Explanation:In a unitary system, regional or local governments only exercise powers delegated to them by the central government.Ghana is a clear example of a unitary state.Nigeria, India, and the United States of America are federal states because their powers are shared between central and regional governments.Therefore, the only unitary state among the given options is Ghana.Answer:D. Ghana