The correct option is
D Exponential phase
The process of obtaining the desired foreign gene product from a genetically engineered organism is known as bioprocessing. It consists of upstream processing and downstream processing.
The biosynthetic phase of growing the genetically modified cells in bioreactors at a large scale is known as upstream processing.
The type of upstream processing where recombinant cells are grown in culture conditions maintained at a steady state is known as continuous culture.
It is characterised by draining out of used culture medium from one side through outlets of the culture vessel/ bioreactor and simultaneous addition of fresh medium from the other side through inlets.
Such a culture method ensures that there is continuous supply of nutrients and the cells maintain exponential growth in the culture.
A typical growth curve shows the following distinct phases of growth.
The lag phase is the initial phase in the growth curve of an organism during which the growth rate is very low as the organisms get accustomed to the new environment. Low concentration of certain nutrients or growth factors can also lead to a lag phase.
After considerable time of inoculation, the cells get adapted to the new environment enriched in growth promoting factors and conditions. This makes them grow rapidly and the phase is termed as log phase or exponential phase. As the number of cells in the culture media increases exponentially, the amount of desired protein production also increases.
The stationary phase is attained in any culture medium when the growth ceases, however, the cells remain metabolically active. This happens when the nutrients in the medium get exhausted.