Nucleus of the cell stores genetic information. The dark structure present within the nucleus is called “nucleolus.” The main function of nucleolus is to synthesise ribosomal RNA (rRNA). Ribosomal RNA joins with the proteins and forms the ribosomal subunits. Ribosomal subunits then enter into the cytoplasm, where they assume their role in protein synthesis.
The main function of mitochondria is energy production. And so, because of this function, they often called as “power houses of the cell.” Depend upon the energy needs; their occurrence varies from cell to cell. For example, skeletal muscles have more mitochondria; whereas red blood cells are completely lack of mitochondria.
Chloroplast is the organelle of plant cells and it contains chlorophyll. Its size is 1 µm so; it can be seen with both light microscope and electron microscope. This organelle is going to generate energy in plant cells.
Thus, the options (b), (c) and (d) are incorrect.
The main function of mitochondria & chloroplast is energy production.
Therefore, the correct option is – a. mitochondrion and chloroplast.