The correct answer is D.
Legislative or Congressional oversight refers to the surveillance performed by the US Congress on the Executive power (government) and all the agencies that conform it. Such surveillance is generally conducted by the Congress comitees and it includes the review and monitoring of programs, and policy implementations.
This Congressional power is a mechanism embodied within the System of Checks and Balances. The System of Checks and Balances is designed to preserve the division of powers between the three branches: legislative (Congress), executive (Government) and judiciary (courts system), so that none of them gathers enough power to overrule the others. With this aim, each of the branches is granted powers (expressed or implicit) to supervise the other two.