Thank. I apologize for not being clear in initial post. I thought the ranking order was A>C=D>B. This is not the answer key. Can you give a more detailed derivation of $$\Delta{\theta} = \frac{\lambda}{Nd}$$
The ranked order from most to least effectively reducing the widths of the primary maxima is: A>D=C>B. I know that doubling the wavelength of incident light will double the width (y) of the fringe. This means that doubling the frequency of incident light will reduce the fringe width (y) by half...