"It is possible to produce transgenic mammals that produce a desired protein product in their milk. After the creation of such a transgenic animal, how would scientists most likely go about developing a whole herd of these transgenic animals"?

Explanation:

Based on the information provided it can be said that most effective way of developing a whole herd of these transgenic animals would be to first  remove the transformed cells from the animal and inject their nuclei into enucleated donor eggs. Once this is done then they can implant them in surrogate mothers to create the transgenic animal.

the answer is:

carbon dioxide