To meet the rapidly rising market demand for fish and seafood, supplie

It seems you didn’t quite understand how to split a sentence into its clauses. I would strongly recommend you to revisit the basics of clauses and get a firm grasp on identifying clauses before proceeding any further.

Having said that, let me break the given sentence for you.
To meet the rapidly rising market demand for fish and seafood, suppliers are
o growing fish twice as fast as their natural growth rate,
o cutting their feed allotment by nearly half and
o raising them on special diets.

The given sentence contains only a single clause. However it contains a list as shown.

Now let's discuss the errors in this sentence:
Error 1
Now observe this sentence.
Suppliers are growing fish twice as fast as their natural growth rate.
This sentence has incorrect elliptical usage. This sentence can actually mean either of the following two:

a. Suppliers are growing fish twice as fast as suppliers are growing their natural growth rate.
b. Suppliers are growing fish twice as fast as their natural growth rate is growing fish.

But both these sentences do not make sense and are illogical.

Consider the following simple examples to get an idea on ellipsis.
i. Mike runs faster than a horse (Mike runs faster than a horse runs)
ii. George likes milk better than coffee. (George likes milk better than he likes Coffee)

Now compare these correct usages with the incorrect elliptical usages of the given sentence mentioned above. You can clearly see that the ellipsis used in the given sentence doesn’t make sense. This is a major error in the given sentence.

Error 2
Also, there is another usage error in the sentence.
Observe that “growth rate” is described as “fast”
Let us take a parallel for better understanding

Rate = speed
Now which one of the following do you think is correct?

Speed is fast. OR
Speed is more/higher.

Saying “speed is fast” is definitely incorrect.
“Speed is more than something” or “Speed is higher than something” is appropriate
On similar lines, growth rate cannot be faster. Growth rate can be higher.
This is another error in the given sentence.

Error 3
Coming to the pronoun ambiguity, lets first apply our pronoun framework:

To meet the rapidly rising market demand for fish and seafood, suppliers are growing fish twice as fast as their natural growth rate, cutting their feed allotment by nearly half and raising them on special diets.

1: What is the logical antecedent of "their", "they"?
Logically these pronouns must refer to "fish".

2: Are there any other nouns in plural (since the pronouns are plural), that could be referred to by these pronouns?
Yes, these pronouns could refer to "suppliers". Thus, there appears to be ambiguity.

3: Is there any answer choice that solves this ambiguity?
No, all answer choices have similar structure.

Thus, in this question, pronoun ambiguity is acceptable to a certain degree. This question exemplifies the fact that pronoun ambiguity rules are hazy and we must not eliminate answer choices right away based on this error. We must first eliminate the answer choices based on other more definite error types - SV, verb, parallelism, modifiers

You can read the framework for “Pronoun Ambiguity” here.
You can find references to OG Questions dealing with Pronoun ambiguity in the following links
Link 1, Link 2

Concluding Remarks
Your explanations for eliminating the other options can be less ambiguous and more detailed. So with the above sentence structure in mind, I would like you to solve the question again and come up with the solution.

Regards,
Krishna