The money multiplier can be calculated using the formula:
\[ \text{Money Multiplier} = \frac{1}{\text{Reserve Ratio}} \]
Given that the reserve ratio is 4 percent (or 0.04 in decimal form), we can plug this value into the formula:
\[ \text{Money Multiplier} = \frac{1}{0.04} = 25 \]
Thus, the money multiplier is 25.
The correct answer is: a. 25.