Look at what physically drives each type of water retention or what variable each method actually measures.
Hygroscopic water clings to particle surfaces through adsorption, a molecular level attraction between the mineral surface and water molecules, so it pairs with adsorption forces.
Capillary water sits in the narrow pore spaces of soil, held there by the same surface tension effect that lets a thin tube draw water up against gravity, so it pairs with surface tension.
A neutron probe emits fast neutrons that lose energy mainly by bouncing off hydrogen atoms, and since water is the chief hydrogen source in soil, the detector count reflects the number of hydrogen nuclei nearby.
The Blaney-Criddle formula for reference evapo-transpiration multiplies a crop coefficient by a factor built from mean monthly temperature and daytime hours, so mean monthly temperature is its defining input.
Assembling these pairs gives (A) with adsorption forces, (B) with surface tension, (C) with hydrogen nuclei count, and (D) with mean monthly temperature.
\[\boxed{(A)\text{-}(III),\ (B)\text{-}(IV),\ (C)\text{-}(I),\ (D)\text{-}(II)}\]