Answer
Ultraviolet radiation.
Work Step by Step
Ultraviolet light has a shorter wavelength than visible light. 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, so using a short wavelength is desirable.
In other words, when this short wavelength radiation passes through the apertures in Hubble’s instruments, the central maximum will be narrower and a higher resolution is attainable.