What happens to the distance between interference fringes if the separation between two slits is increased?

It will decrease

Explanation:

The double-slit interference equation, which gives us the position y of a maximum on the screen (measured relative to the central position), is

where

n is the order of the maximum

D is the distance between the slits and the screen

is the wavelength of the light

d is the separation between the two slits

From the formula, we see that y is inversely proportional to d: this means that if the separation between the slits is increased, the distance of each maximum from the central position (y) decreases, therefore the distance between the interference fringes will decrease.