Answer: condenstation.
Justification:
The polymerization by condensation is a well know chemical reaction in which two monomers ("small" molecules), each with (at least) two functional groups, combine and relase water as by-product. Actually, even if the by-product released is not water, yet the reaction is called condenstation, since the mechanism is basically the same.
An example of such reaction is the manufacturing of nylon 6,6, which is produced from adipic acid and 1,6-diamine hexane:
HOOC - [CH₂]₄ - COOH + nH₂N - [CH₂]₆ - NH₂ → - nylon - + nH₂O
I omitted the formula of nylon because it is large, and that is not the core of the question but the fact the kind of reaction: two molecules combine to form is a larger molecule, and water is released
and any get of units that has those units in the proper place can be considered a density unit. The ones that has those specifically are A, B, E and F