The money multiplier can be calculated using the formula:
\[ \text{Money Multiplier} = \frac{1}{\text{Reserve Ratio}} \]
If the reserve ratio is 25 percent (or 0.25 in decimal form), the calculation would be:
\[ \text{Money Multiplier} = \frac{1}{0.25} = 4 \]
Therefore, the correct answer is:
b. 4.