If the wavelength of the light is increased while keeping the values of a (distance between slits) and D (distance to the screen) constant in a double-slit interference pattern, the maximum brightness of the fringes will not change.
The brightness of the fringes in an interference pattern is determined by the intensity of the light, which is related to the square of the amplitude of the wave. The amplitude of the wave is not directly affected by changes in wavelength.
Therefore, increasing the wavelength of the light while keeping a and D constant will not have any impact on the maximum brightness of the fringes.