Be a fine sharp slit white in colour at the centre.

When light is passing through a slit, diffraction phenomena is only observed when the size of the slit is comparable to the wavelength of light.

In the given question, the slit width is 10⁴ A or 10³ nm and the range of the visible spectrum is 380 nm (Violet) to 700 nm (Red). Hence both are not comparable in nature, therefore, diffraction will not occur hence a fine sharp slit white in colour will be formed due to the mixing of all colours.