The value of the multiplier (k) can be calculated using the formula:
\[ k = \frac{1}{1 - MPC} \]
Given that the marginal propensity to consume (MPC) is 0.75, we can substitute it into the formula:
\[ k = \frac{1}{1 - 0.75} \] \[ k = \frac{1}{0.25} \] \[ k = 4 \]
Therefore, the value of the multiplier is b. 4.