Answer
a. Decreasing the wavelength would improve the resolution.
c. Increasing the lens diameter would improve the resolution.
Work Step by Step
We can write an equation for the resolution:
$\theta_{min} = \frac{1.22~\lambda}{D}$
a. Decreasing the wavelength causes $\theta_{min}$ to decrease. Therefore, decreasing the wavelength would improve the resolution.
b. The resolution does not depend on the focal length.
c. Increasing the lens diameter causes $\theta_{min}$ to decrease. Therefore, increasing the lens diameter would improve the resolution.
d. The resolution does not depend on the magnification.
