Nucleus

The nucleus contains a ground substance and one or more spherical, densely staining bodies that are not bounded by any membrane. This body, the nucleolus, is the site of active ribosomal RNA synthesis and the assembly of ribosomal subunits, a role described in NCERT Class 11, Chapter 8 (Cell: The Unit of Life); cells engaged in vigorous protein synthesis usually possess large or multiple nucleoli. The nuclear envelope is incorrect because it is the double membrane that separates the nuclear contents from the cytoplasm and merely controls exchange across the boundary. Nucleoplasm refers to the transparent, semi-fluid matrix of the nucleus in which the chromatin and the nucleolus are suspended, so it is the medium rather than the rRNA factory. The nuclear pore complex consists of openings in the envelope that regulate the movement of molecules between nucleus and cytoplasm, again unrelated to rRNA production. A logical consistency check confirms the answer: ribosomal subunits must be assembled before export to the cytoplasm, and the structure dedicated to that task is the nucleolus.