Official Explanation:
Read the Original Sentence Carefully, Looking for Errors:
The word "unlike" tells us that this sentence is creating a comparison. When two items are being compared, make sure they are logically similar and in parallel form. The sentence as written compares the battalion's bravery to the bravery ("that of") its Russian counterparts, which meets the requirements of a proper comparison. Presume (A) is the right choice, but look through the other choices to confirm that are no other errors that were overlooked.
Scan and Group the Answer Choices:
(B) and (C) create an invalid comparison between the battalion's bravery and its Russian counterparts, and may be eliminated immediately. It is illogical to compare bravery to people. (A), (D), and (E) all retain the correct phrase to compare the battalion's bravery to “that of its Russian counterparts” and should therefore be examined more closely.
Eliminate Wrong Answer Choices:
The phrase “for fighting” in (D) is ambiguous; exactly who fought off the Germans? Additionally, "like that of..." is stronger than the original "not unlike that.." and therefore changes the meaning of the sentence slightly. (E) uses an incorrect construction to make the comparison. It's correct to say "was like that of" or, in this case, "was not unlike that of"; it's never correct to say "was as that of."
(A) is correct.
TAKEAWAY: Comparisons are often tested on the GMAT. Look out for them, and take a moment to be clear on what exactly is being compared. Make sure the comparison is between logically similar items, and that the items are described in parallel form.