The correct option is D Nucleolus
The rRNA is the form of RNA which is transcribed in the nucleolus. The nucleolus is a structure in the nucleus which is the site for the formation of ribosomes. The rRNA molecules are translated and the subunits are arranged to form the ribosomal structures. So, there is clustering of many molecules of the rRNA in the nucleolus. These ribosomes are then transported to the cytoplasm or the surface of ER. Thus, the correct answer is option D.