Explanation

(a) Width of central maximum is given by
$w = \frac{2f\lambda}{a}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ldots(i)$
Where $f$ is focal length of lens, a is width of slit and $\lambda$ is wavelength of light used.
From Eq. (i), it is clear that fringe width
$\ \ \ \ \ \ \ w \propto \lambda$
So, when blue light is used in the experiment instead of red light, the fringes will become narrower.