While industry insiders insist that fish farming takes a burden off

That comma doesn't make much difference. It just adds a pause. Regardless of whether that comma is there, the phrase beginning with "by" clearly modifies the actions expressed by the words that directly precede the phrase.

In many cases, commas don't make much difference, and their usage is optional.


Here's choice (B).

B. do more harm than they help, by increasing the spread of diseases, the spread of parasites such as sea lice, and astronomically increasing the level of pollution in wild ecosystems

There are two clear issues with that choice.

One is the meaning conveyed by "do more harm than they help." Read literally, that wording conveys that the fish farms do more harm than the harm that they help.

The other issue is that the list in the phrase that starts with "by" is illogical.

The list begins with "by increasing the spread of diseases". Since the second element is "the spread of parasites such as sea lice," the second element needs "by increasing" in order to make sense. So, the second element is understood to be combined with "by increasing" to express "by increasing the spread of parasites such as sea lice."

OK, if "by increasing" carries over to the second element, it carries over to the third as well. We don't carry over to some elements of a list and not to others. So the third element is understood to be combined with "by increasing" as well, with the result that we get the illogical "by increasing astronomically increasing the level of pollution in wild ecosystems."