The correct option is D All of the above
Growth, an irreversible process, occurs at all levels, i.e., at cell level, subcellular level as well as organ level. Cell level growth means the cells multiply and increase in number and size. The organ level means there is an increase in the size of organ with time and subcellular level means the increase in number and/or size of subcellular organelles, like mitochondria, endoplasmic reticulum etc.