Zoologist: Every domesticated large mammal species now in existence was domesticated thousands of years ago. Since those days, people undoubtedly tried innumerable times to domesticate each of the wild large mammal species that seemed worth domesticating. Clearly, therefore, most wild large mammal species in existence today either would be difficult to domesticate or would not be worth domesticating.
Type- assumption
conclusion - most wild large animals we see today either wouldn't be worth domesticating or would be too difficult to domesticate.
(A) in spite of the difficulties encountered, at one time or another people have tried to domesticate each wild large mammal species- incorrect, 'each' is a bit extreme, The author merely said that early humans tried to domesticate each wild mammal worth domesticating.
(B) it is not much easier today to domesticate wild large mammal species than it was in the past - Correct, If you negate (B), it says "it IS much easier today to domesticate wild large mammals" than it used to be. Is this an objection to the argument?
Yes. If it's much easier to domesticate wild mammals now, then some species deemed "too hard to domesticate" by early humans would now be viable candidates for domestication. That would go against the author's conclusion.
(C) not all of the large mammal species that were domesticated in the past are still in existence - incorrect, its wording is close to a legit assumption - the author is assuming that "all of the undomesticated large mammal species in existence today were in existence in the past". The author thinks that all the wild mammals we see nowadays have been evaluated by early humans
(D) the easier it is to domesticate a wild large mammal species, the more worthwhile it is to do so - incorrect, Too extreme. Be careful of "the more X, the more Y" type answers. The author doesn't need a relationship between "how easy" and "how worthwhile". He believes early humans evaluated "how worthwhile" and "how easy".
(E) of all the domesticated large mammal species in existence today, the very first to be domesticated were the easiest to domesticate - too extreme
Answer B