Constitutional Government means ​

Constitutional government means a government limited by the terms of the constitution, so D is correct. This is the core idea of constitutionalism: state power is bounded by fundamental law, and rulers cannot act beyond it. In India this shows up in Article 13, which voids laws inconsistent with Part III, and in judicial review, which lets courts test executive and legislative action against the Constitution. The basic structure doctrine in Kesavananda Bharati (1973) is the sharpest example of a limit even on Parliament's amending power.

• A is wrong. Federalism is a division of power between levels of government. A unitary state can also be constitutional, so federal structure is not the defining feature.
• B is wrong. A nominal head describes the form of a parliamentary or constitutional monarchy. Such systems are constitutional, but nominal headship is not what makes a government constitutional.
• C is wrong. A head with real powers, as in a presidential system, can still be bound by a constitution. Real versus nominal power is a question of structure, not of limitation.

Mains connection: GS-II topics on separation of powers, judicial review and the basic structure doctrine all rest on this idea.

Trap: aspirants pick A or B by linking the word 'constitutional' to parliamentary or federal forms. Ask what limits power, not how power is shared.