The main function of the nucleolus is option D: it is the site of ribosome subunit assembly.
The nucleolus is a specialized structure within the nucleus of eukaryotic cells. Its primary role is to synthesize ribosomal RNA (rRNA) and combine it with proteins to form ribosomal subunits. These subunits are then transported out of the nucleolus and into the cytoplasm, where they combine to create functional ribosomes, which are essential for protein synthesis within the cell.
In contrast, options A, B, and C describe functions that do not relate to the nucleolus. While cell adhesion proteins are important, they are not produced by the nucleolus. The function of the nucleolus is well understood, and it does not play a role in photosynthesis, which occurs in chloroplasts. Therefore, the correct and most accurate description of the nucleolus’s role is its participation in ribosome assembly.