The correct option is
A Bubbles
Bioreactors are large vessels that are used for large scale culturing of cells to obtain specific gene products for commercial use.
A bioreactor provides optimum growth conditions, such as, temperature, pH, nutrients, oxygen (aerobic bioreactors), etc, for the growth of cells in order to achieve the desired gene product.
One of the most commonly used bioreactors is stirred tank type bioreactors. This is engineered to ensure stirring of contents in the culture medium thoroughly. These can further be of two types - simple stirred-tank bioreactors and sparged stirred-tank bioreactors.
A sparged tank bioreactor is an aerobic bioreactor which is characterised by increased surface area availability for aeration of culture medium. The surface area permits air above the liquid culture to get trapped in the form of bubbles which then travel throughout the culture. This is also known as gas entrainment. It allows diffusion of oxygen into the culture. Sterile bubbles are also sparged into the culture medium for aeration. The bubbles increase the surface area for oxygen transfer.

With time, the turbidity of the culture medium increases due to the increase in the number of cells growing in it and the release of their metabolic products and wastes. Turbidity refers to the state of being clouded or opaque.
The turbidity, the cells in the culture media or the proteins released by them have no role in increasing the oxygen transfer area.