Answer
See explanation.
Work Step by Step
As explained on page 1194, equation 36.11 characterizes the width of the central maximum: $\theta_1 = \frac{\lambda}{a}$.
A smaller angle means a decreased width of the central maximum.
a. Decreasing a, by using a narrower slit does not decrease the width of the central maximum.
b. Using light of a smaller frequency raises the wavelength, which does not decrease the width of the central maximum.
c. Lowering the wavelength does decrease the width of the central maximum.
d. Moving the screen closer to the slit does not affect the angular width of the central maximum, but since the screen is closer, the pattern width on the screen will decrease. This is also apparent from equation 36.3.
