‘A cell within a cell’ is applicable to
A. Mitochondria
B. Chloroplast
C. Virus
D. Both A and B.

Hint:A cell is a mass of cytoplasm that is bound externally by a cell membrane. A cell consists of various cell organelles within it.

Complete answer:
To answer this question, first, we need to know about the cells. Cells are recognized as the basic blocks of life because they come in distinct and easy identifiable form which is because cells are enclosed by a shape known as the cell membrane that more similar with the walls of a house, functions as a distinct bar between the inner and outer environments of the cells. The cell membrane is sometimes also referred to as the plasma membrane.
Now, let us find the solution from the option.
Mitochondria and chloroplasts are unique cell organelles in a eukaryotic cell. Both are double layered membrane; consisting of DNA, protein synthesizing machinery and ribosomes. They are semi-autonomous entities. The ribosomes of these organelles resemble prokaryotic ribosomes. These characteristics are seen as evidence supporting the endosymbiotic theory. The endosymbiosis theory mentions that mitochondria and chloroplasts are the result of development starting by the endocytosis of blue - green algae and bacteria. Based on this theory, blue green algae and bacteria were not dissolved; they form symbiotic instead of that. This concept has been expressed by the phrase, 'A Cell within a cell'.
 A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism.

Thus, the correct option is D. i.e. Both A and B.


Note: Cells were discovered by Robert Hooke in 1665, who mentioned them for their similarity to cells stopped by Christian monks in a monastery. Cell theory, first modified in 1839 by Matthias Jakob Schleiden and Theodor Schwann, mentioned that all organisms are consisted of one or multiple cells, those cells are the basic block of function and structure in all living beings, and in that way all cells produced from pre-existing cells.