What is a central bank primarily responsible for?

A central bank is primarily responsible for regulating the country's currency. This includes managing the nation's money supply, controlling inflation, and ensuring monetary stability. The central bank implements monetary policy through various tools such as interest rates, open market operations, and reserve requirements. By controlling the currency and by extension the country's monetary policy, the central bank plays a crucial role in stabilizing the economy, influencing employment levels, and fostering economic growth.

The other options, while relevant to the financial system, do not encompass the primary role of a central bank. Managing private banks is often a function of the central bank, but it's not its primary responsibility. Similarly, overseeing stock exchanges and controlling government expenditures are typically managed by different regulatory bodies and government departments, rather than the central bank itself.