I would go with E

In Scotland, the number of wild salmon have been reduced because of uncontrolled deepsea and costal netting, by pollution, and by various other threats to the fish’s habitat.

A. number of wild salmon have been reduced because of uncontrolled deep-sea and coastal netting - number...have (incorrect)
B. number of wild salmon is reduced because deep-sea and coastal netting is not controlled - seems like deep-sea and netting is not controlled by pollution
C. numbers of wild salmon has been reduced because of uncontrolled deep-sea and coastal netting - numbers...has
D. wild salmon’s numbers are reduced by deep-sea and coastal netting that is not controlled - same problem as B
E. wild salmon’s numbers have been reduced by uncontrolled deep-sea and coastal netting - correct answer...

mehravikas wrote:In Scotland, the number of wild salmon have been reduced because of uncontrolled deepsea and costal netting, by pollution, and by various other threats to the fish’s habitat.

A. number of wild salmon have been reduced because of uncontrolled deep-sea and coastal netting - number...have (incorrect)
B. number of wild salmon is reduced because deep-sea and coastal netting is not controlled - seems like deep-sea and netting is not controlled by pollution
C. numbers of wild salmon has been reduced because of uncontrolled deep-sea and coastal netting - numbers...has
D. wild salmon’s numbers are reduced by deep-sea and coastal netting that is not controlled - same problem as B
E. wild salmon’s numbers have been reduced by uncontrolled deep-sea and coastal netting - correct answer...

Thanks for the explanations!

ranell wrote:In Scotland, the number of wild salmon have been reduced because of uncontrolled deepsea and costal netting, by pollution, and by various other threats to the fish’s habitat.

A. number of wild salmon have been reduced because of uncontrolled deep-sea and coastal netting
B. number of wild salmon is reduced because deep-sea and coastal netting is not controlled
C. numbers of wild salmon has been reduced because of uncontrolled deep-sea and coastal netting
D. wild salmon’s numbers are reduced by deep-sea and coastal netting that is not controlled
E. wild salmon’s numbers have been reduced by uncontrolled deep-sea and coastal netting

E maintains parallelism and correct verb tense....
by uncontrolled...by pollution...by various...
also numbers have been reduced..is correct

In Scotland, the number of wild salmon have been reduced because of uncontrolled deepsea and costal netting, by pollution, and by various other threats to the fish's habitat.

A. number of wild salmon have been reduced because of uncontrolled deep-sea and coastal netting
----> "by" is missing that can make the sentence parallel + "number--have" is wrong.
B. number of wild salmon is reduced because deep-sea and coastal netting is not controlled
----> "by" is missing that can make the sentence parallel + "number" is reduced is wrong again.
C. numbers of wild salmon has been reduced because of uncontrolled deep-sea and coastal netting
----> "by" is missing that can make the sentence parallel + "numbers..has" is incorrect.
D. wild salmon's numbers are reduced by deep-sea and coastal netting that is not controlled
-----> "that is not controlled" is wordy
E. wild salmon's numbers have been reduced by uncontrolled deep-sea and coastal netting
----> is parallel for it has "by" + it matches numbers with have + correct tense is used

There's nothing incorrect about the phrase "wild salmon's numbers" ?

For E, is there anything wrong with "the wild salmon's numbers"? Does the phrase refer to "a variety of wild salmon"? Except for this seemly abnormal phrase, I find nothing wrong about E?

Isnt 'The number' singlular so it needs a singular verb like 'is' /'has' which is present in B??

Yes " wild salmon's numbers" is not a correct phrase but using the rule "In GMAT Best answer doesn't mean Ideal answer choice".. E stands apart from other choice with minimum flaw... I would say "wild salmon's " not that wrong because salmon is an animate subject so it can take the possessive form.

Points to note -

1. A number of -Plural
2. The number of - singular
3. Numbers of - always wrong

D vs E

The 'THAT' (essential modifier) in D modifies only coastal netting and hence changes the meaning. Both deep sea and coastal netting are uncontrolled, leading to the drop in numbers.

IMO:E