The correct option is C Viruses
In 1921, Banting and Best first isolated insulin from fetal calves. Later, pigs and cows became the two major sources of insulin. This was at the time when genetic engineering was yet to be developed and insulin for humans was extracted from animal sources. This was extremely expensive and the quantity was very low.
Now, genetic engineering has enabled us to produce mammalian insulin from GM bacteria.
Viruses cannot be grown in synthetic medium and hijack the host splicing machinery to process their RNA segments hence they cannot be used for the production of biological products.