Answer
(a) The width of the central maximum increases.
(b) The width of the central maximum decreases.
(c) The screen would have a uniform color with no apparent rings.
Work Step by Step
We can write an equation for width of the central maximum:
$w = \frac{2.44~\lambda~L}{d}$
(a) If the wavelength $\lambda$ is increased, then the width of the central maximum increases.
(b) If the diameter $d$ of the aperture is increased, then the width of the central maximum decreases.
(c) $sin~\theta = \frac{n~\lambda}{d}$
If the aperture diameter $d$ is less than the light wavelength, then $sin~\theta \gt 1$, which means that no dark fringe would appear. The screen would have a uniform color with no apparent rings.
