Organelle within organelle means an organelle present in another organelle
As we know ribosomes present in matrix of mitochondria and stroma of chloroplast,we can consider them as organelle within organelle

Answer:
An organelle within an organelle refers to a structure that is present inside another organelle within a eukaryotic cell. Among the given options, the correct answer is option 'A', which represents the ribosome.

Explanation:
To understand why the ribosome is an organelle within an organelle, let's discuss the structure and function of ribosomes and the organelle in which they are found, i.e., the endoplasmic reticulum (ER).

Ribosomes:
- Ribosomes are small, spherical structures found in both prokaryotic and eukaryotic cells.
- They are responsible for protein synthesis, converting genetic information from the messenger RNA (mRNA) into functional proteins.
- Ribosomes can be found freely floating in the cytoplasm or attached to the endoplasmic reticulum.

Endoplasmic Reticulum (ER):
- The endoplasmic reticulum is a complex organelle found in eukaryotic cells.
- It is involved in various cellular processes, including protein synthesis, lipid metabolism, and calcium storage.
- The ER is composed of a network of interconnected tubules and flattened sacs called cisternae.
- There are two types of ER: rough ER (RER) and smooth ER (SER).

Ribosomes within the ER:
- The rough endoplasmic reticulum (RER) is studded with ribosomes on its outer surface.
- These ribosomes are attached to the ER membrane and are involved in protein synthesis.
- As the ribosomes synthesize proteins, they thread the growing polypeptide chain into the ER lumen (inner space of ER).
- This allows the newly synthesized proteins to enter the ER for further processing, modification, and transport to their final destinations within the cell or outside the cell.

Conclusion:
In conclusion, the ribosome is an organelle within an organelle because it is located within the endoplasmic reticulum. The ribosomes attached to the ER membrane play a crucial role in protein synthesis and secretion. Thus, option 'A' (Ribosome) is the correct answer.