A) -> Whether it thrives in other soil or not is not relevant to finding out if it thrives in metal rich soil because of histidine. This option is out of scope to what's asked.
(B) -> If true, will give us confidence that histidine is indeed a critical component of surviving the toxic metal rich soil. This would indicate that the mentioned herb has something common with other plants that thrive in the patch. Most likely, if this is the case, the common factor is the one that helps them survive.
(C) -> Irrelevant. Consider two cases - A. some other amino acid is produced in large quantities - it may be a compound not related to toxic survival (it could be something that causes leaves to grow bigger, for example). B. some other amino acid is produced in unusually low quantities - this does not help the case of plant surviving because of histidine. What other acids do, or their quantities is not helpful in finding if histidine is the key factor for survival.
(D) -> This is tricky. Suppose metal concentration is reduced over time. Also suppose that this is due to neutralizing effect of histidine. All this says is that the plant reduces concentration of metal. Doesn't say anything about whether histidine is the critical factor in survival. It could be anything else. The effect histidine has on the metal concentration may not be key to the plant's survival. Any other factor (say a toxic filtering mechanism in the plant's roots) may be the most critical. So, histidine's effect on soil is not the most helpful in finding out if histidine is THE most important feature that helps survival.
(E) -> Age of the plant is not in scope for the argument and is not needed to find if histidine is the true savior.