Answer
(a) decrease
(b) II
Work Step by Step
(a) We know that $Y_m=m\frac{\lambda D}{d}$. This equation shows that the spacing between the fringes decreases if the color of the light is changed from red to blue because $\lambda_{blue}\lt \lambda_{red}$.
(b) We know that the best explanation is option (II) -- that is, the fringe spacing decreases because blue light has a shorter wavelength than red light.
