The true statement for nucleolus is:
Option D: It is a site for active ribosomal RNA synthesis.

The nucleolus is a distinct region within the nucleus of a cell and is responsible for the synthesis and assembly of ribosomes. It plays a vital role in the production of ribosomal RNA (rRNA) and ribosomal subunits, which are essential for protein synthesis.

Functions of the Nucleolus:
1. Ribosomal RNA Synthesis: The nucleolus is the primary site for the synthesis of ribosomal RNA (rRNA). In the nucleolus, a specific type of RNA polymerase called RNA polymerase I transcribes the genes that encode for rRNA. These genes are organized into repeating units called nucleolar organizer regions (NORs). The rRNA molecules produced in the nucleolus combine with proteins to form ribosomal subunits.

2. Assembly of Ribosomal Subunits: The nucleolus is also involved in the assembly of ribosomal subunits. After the synthesis of rRNA, it combines with ribosomal proteins to form the large and small subunits of ribosomes. These subunits are then exported from the nucleolus to the cytoplasm, where they participate in protein synthesis.

3. Regulation of Ribosome Biogenesis: The nucleolus plays a crucial role in regulating ribosome biogenesis. It controls the number and size of nucleoli present in a cell, depending on its metabolic demands. The nucleolus also acts as a sensor for cellular stress and can halt ribosome production under unfavorable conditions.

4. Storage of Ribosomal Proteins: The nucleolus acts as a storage site for ribosomal proteins, which are necessary for the assembly of ribosomal subunits. These proteins are synthesized in the cytoplasm and transported into the nucleolus, where they are stored until needed for ribosome assembly.

Conclusion:
In conclusion, the nucleolus is not a membrane-bound structure and does not participate in spindle formation. However, it is a site for active ribosomal RNA synthesis and plays a crucial role in the production and assembly of ribosomes, which are essential for protein synthesis in cells.