Answer
Only using light of a higher frequency increases the resolving power.
Work Step by Step
As explained on page 1205, equation 36.17 shows the minimum angular distance between 2 sources that can be resolved: $\theta = sin^{-1}1.22\frac{\lambda}{D}$. A smaller angular distance means higher/better resolving power.
a. Decreasing D, by using a smaller lens, will not increase resolving power.
b. Using light of a higher frequency lowers the wavelength, which does increase resolving power.
c. Raising the wavelength does not increase resolving power.
