One helpful trick for long and confusingly ordered sentences is to remove unnecessary information from the sentence. If part of a sentence is set off by commas, that portion of the sentence should not be grammatically necessary to the sentence. Here, the part of the sentence about and Alaskan brown bears weight and length is set off by commas in all of the answer choices. This means that if we remove everything between the commas, we should still have a grammatical sentence.

Applying this trick, we should be able to eliminate D and E right away - both "It is the largest of all land carnivores" and "the Alaskan brown bear is a close relative of grizzly bear" are independent clauses, so separating them with only a comma is a comma splice.

Unfortunately, we see that the only thing that changes between A, B, and C is the portion of the sentence set off by commas, so we need to look specifically at that part. A starts with "weighing", which is a common start for a dependent clause. We also see that "weighing" and "measuring" are parallel. No apparent issues here.

B breaks the parallelism we see in A, with "weighing ... it measures". We can eliminate B.

C is parallel ("it weighs ... measures"), but it gives us a new independent clause ("it weighs up to 1500 pounds and measures 8 feet in length"), which is only separated from our second independent clause by a comma. This means we have another comma splice. We can eliminate C, leaving us with A as our correct answer.