Which one of the following structure is an organelle within an organelle?

Correct option is A. Ribosome
In eukaryotic cells, 80 S ribosomes (organelles) are attached to the rough endoplasmic reticulum(RER) which is an organelle. It is also present in the organelles: chloroplast (in plant cells) and mitochondria. Thus ribosomes are called organelles within the organelle.Hence Ribosome is an organelle within an organelle.So, the correct answer is 'Ribosome'