Explanation

First impressions: this argument looks long and appears to involve some obscure definition. We can check the question stem: we're asked to strengthen the argument. We can focus on the elements of opinion. The last sentence is the opinion, whereas the first three sentences give facts.

Creating a filter: The key is that the "amount of nutrients" is the same between organic and synthetic fertilizer. Why? Because they are both 4-8-6, meaning they have the same proportions of nutrients. These terms have been matched by the argument. Is the "amount of nutrients" delivered the same because the proportions are the same? Is there any way these things could be different? We'll take that idea our filter for basic relevance.

Applying the filter, we eliminate (A), (C) and (E), and keep (B) and (D). Choices (A), (C) and (E) are irrelevant comparisons--they have no bearing on whether these two fertilizers work identically. Choice (D) starts to get after the same considerations, but it discusses contaminants and nutrients other than the ones at hand, and it doesn't say that the levels are equal, only "no higher" in synthetics. So (D) is not material to the claim in the prompt.

Logical proof: using the negation test, we can see that the negated (B) weakens the argument. If the mechanisms by which organic and synthetic fertilizers are absorbed in soil are substantially different, then the argument is flawed and the conclusion may be incorrect. The non-negated (B) therefore patches a weakness of the argument and does, in fact, strengthen the argument. The correct answer is (B).