Constitutional government means

1. A

   a representative government of a nation with federal structure

   Federal representative government. One form, not the meaning of ‘constitutional’.

2. B

   a government whose Head enjoys nominal powers

   Nominal head. Parliamentary, not the definition.

3. C

   a government whose Head enjoys real powers

   Real-power head. Presidential, not the definition.

4. D

   a government limited by the terms of the Constitution

   A government limited by the Constitution. Constitutionalism.

Summary. Official key is (d). Constitutional government means public power is limited by a constitution — not federalism, and not whether the head is nominal or real.