The label A represents mitochondria.
The mitochondria is also called the powerhouse of the cell which converts glucose to energy rich molecules of ATP.
Label B represents nucleus.
Nucleus contains the genetic material or DNA, RNA , nucleolus, chromatin fibres and proteins. It contains the chromosomes which control the hereditary characteristics.
Label C represents lysosome.
Lysosomes contain digestive juices which breaks down large cell parts and large molecules and helps in intracellular digestion.
Label D represents rough endoplasmic reticulum having ribosomes.
The rough endoplasmic reticulum provides surface for protein synthesis. It is also used to transport cellular contents from one cell to another.