The sentence as written has no grammatical errors. One potential problem would be a lack of parallelism in the two-part predicate "A ... moose ... is likely to be ... and is often unable to protect ...." However, the sentence is in correct parallel form. A glance at the answer choices shows that the question may be testing the choice and placement of "likely" or "liable," but "likely" is correctly used here. However, some of the answer choices would create shorter sentences, potentially reducing wordiness, so check them for errors before choosing (A).

The beginnings of the answer choices are worded in a variety of ways that makes grouping difficult. However, one way to group the answer choices is with the placement of "often" toward the end. (A) and (B) have "often unable to," (C) and (D) move often to the end with "cannot often," and (E) moves it forward with "often likely to be unable that it can."

(B) is the shortest choice and uses "often" in the same position as in the original sentence, preserving its meaning. However, (B) introduces a parallel structure error by removing the word "is" from the second part of the compound predicate. Eliminate.

By moving "often" from before "unable" to before "protect," (C) and (D) subtly change the meaning of the sentence. They also incorrectly replace "is" with "cannot" and introduce parallel structure errors. The "moose ... is liable that it is" in (D) is very awkward to the point of being nonsensical. Eliminate both of these.

The "often likely to be unable that it can protect itself" of (E) is gibberish, and the movement of "likely" from before the idea that the moose is at a disadvantage to before its inability to protect itself subtly changes the meaning of the sentence. Throw out this one.

(A) is correct. The sentence contains no errors as written, and the other choices offer no improvement.