The question stem reads "The zoologist's argument requires the assumption that."
In the video lesson, we learned that "requires" indicates a necessary assumption. A necessary assumption is defined as follows: if the statement is false,then the conclusion would be false.
Answer choice B reads "it is not much easier today to domesticate wild large mammal species than it was in the past."
What if that statement were false? That would mean it IS much easier to domesticate those types of animals today. If that were true, then the fact that such animals were not domesticated long ago would be irrelevant to whether they can be domesticated today. The conclusion would fall apart.