The correct option is A Judiciary
The Judiciary interprets the Constitution as its final arbiter. It is its duty as mandated by the Constitution, to be its watchdog, by calling for scrutiny any act of the legislature or the executive, who otherwise, are free to enact or implement these, from overstepping bounds set for them by the Constitution. It acts like a guardian in protecting the fundamental rights of the people, as enshrined in the Constitution, from infringement by any organ of the state. It also balances the conflicting exercise of power between the centre and a state or among states, as assigned to them by the Constitution.