Hi valentina07!
The conclusion to the stimulus is, "Hence, in all probability, Megatherium was a predator." The stimulus arrives at this conclusion because (1) it has a short olecranon process, (2) the shorter this bone is, the faster forelimbs can typically be moved, and (3) predatory mammals that attack prey quickly have short olecranon processes.
Effectively, this assumes a particular cause-and-effect relationship:
SOP 
P
That is, if a mammal has a short olecranon process, then it is a predator. However, the following also seems possible:
P SOP
In other words, if an animal is a predator, then it has a short olecranon process. Answer choice (A) gets at this. By concluding that Megatherium was a predator, the stimulus "fails to address the possibility that most mammals with short olecranon processes have not been predators," i.e., the stimulus assumes the first rather than the second of these conditionals is true. It's possible that a mammal might have a short olecranon process and not be a predator, but the stimulus does not allow for this.
Answer choice (D) states that the stimulus "takes for granted that, on average, the olecranon processes of extinct mammals were no shorter than those of living mammals are." The stimulus doesn't make claims about "extinct mammals" in general, which puts this answer choice out of contention. The conclusion is specifically about Megatherium and what we can conclude about it based on a particular piece of evidence. Whether or not the olecranon process in extinct mammals generally is shorter than it is in living mammals wouldn't clearly address a conclusion about whether or not the Megatherium was a predator.
