Good question. This is a tough one. It was a very close call between D and E, and I choose E.
In questions like these, where causality is involved, in order to strengthen the argument, we need to prove that the result is ONLY obtained as a consequence of what was stated.
For ex: If A causes B, then
- if we say no other C causes B, then we are strengthening the argument.
- similarly, if we say some other C causes B, then we are weakening the argument.
In this question
A = high histidine
B = nullify metallic affect
Since we need to strengthen here, we need to find out an answer which states (directly or indirectly) that metallic affect was indeed nullified by histidine ONLY
A. histidine is found in all parts of the plant—roots, stem, leaves, and flowers
Not a great strengthener. Moreover histidine present in other parts of the herb is not relevant. Only which is present in roots is relevant
B. the herb’s high level of histidine production is found to be associated with an
unusually low level of production of other amino acids
This is actually a weakener, because it implies that there is another source which could be possible for nullifying metallic affect
C. others of the closely related group of plants are also found to produce histidine in
D. cultivation of the herb in soil with high concentrations of the metals will, over an
extended period, make the soil suitable for plants to which the metals are toxic
This is actually a weakener. It says that over time, the soil itself adjusts itself to the plant, indicating that the soil is the one nullifying the metallic affect, and not the plant.
E. the concentration of histidine in the growing herb declines as the plant approaches
Yes, strengthens the argument. It implies that the plant has been producing histidine for a long time, to nullify the metallic affect, and hence as the plant reaches maturity, the plant's production of histidine has decreased.
If histidine weren't the cause for nullifying metallic affect, the plant would still be capable of producing large amount of histidine even when it is mature