Which one of the following is the site for active ribosomal RNA synthesis ?

The correct answer is Nucleolus.

Ribosomal RNA (rRNA) is transcribed in the cell nucleus, but not in the general chromatin. The specific nuclear substructure where rRNA genes are actively transcribed is the nucleolus.

Here is the reasoning step‑by‑step:

1. Location of rRNA genes
The genes that encode rRNA are repeated many times in the genome and are found in the short arms of the five acrocentric chromosomes. These regions are called nucleolar organizer regions (NORs).

2. Transcription machinery
RNA polymerase I, which is specialized for rRNA synthesis, binds to the NORs and initiates transcription. This enzyme functions only within the nucleolus.

3. Formation of the nucleolus
As transcription proceeds, the nascent rRNA transcripts and associated proteins aggregate, creating the dense, membraneless organelle known as the nucleolus.

4. Processing and assembly
Inside the nucleolus, the primary rRNA transcript is processed (cleaved, chemically modified) and combined with ribosomal proteins to form the ribosomal subunits.

5. Exclusion of other structures
- Chromatin is the general DNA‑protein complex throughout the nucleus and does not specialize in rRNA synthesis.
- Centrosome is involved in microtubule organization, not nucleic acid transcription.
- Kinetochore is a protein structure on chromosomes that attaches to spindle fibers during cell division, unrelated to rRNA production.

Therefore, among the given options, the nucleolus is uniquely dedicated to active ribosomal RNA synthesis.