Explanation

To obtain greater dispersion by a diffraction grating, the slit separation should be decreased. 

Dispersion:

• In diffraction gratings, dispersion is the separation of light wavelengths during diffraction, with greater dispersion resulting in wider color separation.

Slit separation:

• The distance between individual slits on the grating is called the slit separation.

Relationship between slit separation and dispersion:

• Smaller slit separations lead to larger angles of diffraction for a given wavelength, resulting in greater dispersion.
• Imagine the light waves as being "bent" more by the closer slits, causing them to spread out further