The correct option describing the difference between the fiscal deficit and the national debt is:
B) The national debt is a stock concept while the fiscal deficit is a flow concept.
In economics, a stock refers to a measure of value at a specific point in time (i.e., the amount owed at a moment), while a flow refers to a measure observed over an interval of time (i.e., how much is borrowed each year). The national debt is the total amount of money that a government has borrowed and not yet repaid. It is a stock concept because it represents the total outstanding debt at a point in time. On the other hand, the fiscal deficit is the difference between the government's spending and its revenues within a specific period, usually a year. It is a flow concept because it indicates the amount of borrowing that has occurred within a specified time frame.
Therefore, option B is the correct description of the difference between the fiscal deficit and the national debt.
\textbf{Answer: } B