Where is ribosomal $-RNA$ actively synthesized?

What do $a$ and $b$ represent in the given figure,respectively?

The Feulgen reaction is a special test for:

Nucleosomes are bounded by

Identify the cell organelle based on the following characteristics:
- It is not a membrane-bound organelle.
- It is the site for active ribosomal $RNA$ $(rRNA)$ synthesis.
- It is a spherical structure.

The study of nuclear cytology is called: