The Pacific Salmon of the Columbia River exists at just 3% of its

The word being represents the action is/was happening. Almost always the usage of being is wrong in GMAT, except few situations. The proper usage of being in few sentences:
I am being nice.
He was caught being drunk in the back of the cab.
A. on account of being killed by a parasite - who is being killed? Here we are looking at cause and effect, the fish is being killed and that is why their population shrinks.
B. on account of their being killed by a parasite - If you read the sentence, it says its original population level. Remember fish can either be plural or singular, based on the context. From this sentence, we could infer that fish is singular. So, the pronoun their does not have any antecedent.
C. because it is killed by a parasite - Now the pronoun refers to the fish and we got to know what is being killed!
D. because of being killed by a parasite - Who is being killed? Again, there is no link between cause and effect
E. being that they are killed by a parasite - Pronoun error as in B.

Don't eliminate the answer choices just because the answer choice has being. Yes, most of the times the answer choice is wrong, but try for other options to eliminate too.

elavendan1 or anyone “being” is also used to represent temporary states, here how do we know that the killing isnt a temporary state i.e. the killing will stop once the parasite stops transferring, because earlier the population was greater.

Also I get that “because” needs a subject after it but do “on account of” also needs a subject?

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Answer in 10 secs...On account of is mostly wrong and the use of being is not accepted in gmat.

D is wrong.

Because of takes a noun as we have a preposition of, because of should be followed by a noun or a noun phrase for example Avicci died in is prime because of drug overdose or we can say Avicii died in his prime because he met with an accident.
If the cause is because of noun use because of. If the cause is because of some action use because followed by a clause.
I do not see problem with being here. They are being killed is a correct sentence. D has far more grave error meaning.

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