Colour in transition metal compounds is attributed to

Incomplete (n–1)d subshell

The answer is (4), incomplete (n-1)d subshell.

Transition metal compounds are coloured because of the presence of unpaired electrons in the (n-1)d subshell. When light is shone on these compounds, the unpaired electrons can absorb photons of light with the same energy as the difference in energy between the two d orbitals. This causes the electrons to be excited to the higher energy d orbital, and the compound appears coloured.

The other options are incorrect.

Option (1), small size metal ions, is incorrect because the size of the metal ion does not affect the color of the compound.

Option (2), absorption of light in UV region, is incorrect because transition metal compounds do not absorb light in the UV region. They absorb light in the visible region, which is why they appear coloured.

Option (3), complete (n, s) subshell, is incorrect because transition metal compounds must have at least one unpaired electron in the (n-1)d subshell in order to be coloured.