A certain cultivated herb is one of a group of closely related plants

jabhatta2

Hmm. For one thing, I can follow your reasoning. For another, (C) was quickly dismissed when I attempted this question.

The prompt provides at least a plausible reason why large amounts of histidine might help herb grow in soil with high concentrations of metals: because histidine, in test-tube solutions, renders these metals chemically inert.

(C) is not only vague (an unusually low level of production of some other amino acid) but also silent on why those other amino acid (, which have to be at unusually low level) can help the herb survive and thrive. If the level is unusually low enough to approach nonexistent, any plants that do not produce the the amino acid should be immune to the toxic metal as well, contradicting the prompt.

If you happen to know that there are only 20 amino acids altogether, so their attributes are intensively studied, you may find (C) even less appealing.

For "evaluating the argument", a common approach is to answer "Yes" and "No" to the proposed question and check its impact on the conclusion.
Is the herb's high level of histidine production associated with an unusually low level of production of some other amino acid?
"Yes" (Let's interpret "associated with" as "concurrently with") It should have no impact on the conclusion. We can almost always find an unusually low level of some other amino acid.
"No" It also has no impact on the conclusion. It is possible all other acid are not "usually low level". We do not really know the definition of "usually low".