Final answer:
Tax revenues rise, indicating the nation is on the upward-sloping portion of its Laffer curve. Therefore, the correct option is A.
Step-by-step explanation:
When the flat income tax rate in a certain nation decreases from 70 percent to 65 percent and the tax base increases from $500 billion to $575 billion, it signifies that tax revenues rise. This indicates that the nation is on the upward-sloping portion of its Laffer curve.