National Banking Act is a decision, that is available to all civilized nations after they have researched Modern Central Banking System and have at least 1000$ in cash.
It is visible once a nation has researched Central Bank Money Bill Printing.
The decision will reduce the loan interest by 2% for the rest of the game for the rest of the game, at the cost of 1000$. It is worth it to take, if the player expects that the country will loan more than 50.000$ during the rest of the game.