The Basel III reforms introduced a leverage ratio buffer specifically for global systemically important banks (G-SIBs). This buffer is designed to ensure that G-SIBs maintain additional capital to absorb losses and reduce the risk of their failure, which could have significant systemic consequences. The leverage ratio buffer is part of the broader Basel III framework aimed at strengthening the regulation, supervision, and risk management of the banking sector.

Key Points:

• Option A (global banks) is too broad, as the buffer applies specifically to systemically important banks, not all global banks.
• Option B (unregulated global banks) is incorrect because the buffer applies to regulated G-SIBs.
• Option D (banks regulated by the U.S. Federal Reserve and the European Banking Authority) is too narrow, as the buffer applies to G-SIBs globally, not just those in specific jurisdictions.

Thus, the correct answer is C.