The correct answer is: "a bank"
The US Securities and Exchange Commission (SEC) is an independent agency of the US federal goverment which is in charge of enforcing the federal legislation related to securities, of proposing new legislation and of monitoring the stock and options markets all over the country and the securities industry (including electronic securities), etc.
Therefore, this supervision activities entail monitoring the activity of the banking sector, as banks are important stakeholders in the securities industry, and the penalization of those which are not complying with the existing regulations.