Answer:
halve the slit separation
Step-by-step explanation:
As we know that
In YDS experiment, the equation of fringe width is as follows

where,
D denotes the separation in the middle of screen and slits
d denotes the distance in the middle of two slits
And to increase the Δx we have to decrease the d i.e, the distance between the two slits
Hence, the first option is correct