Answer:
.D) The Malbec vine is susceptible to phylloxera, a plant louse that only grows at low altitudes.
Step-by-step explanation:
What weakens the conclusion that Argentinian winegrowers should grow the Malbec grape at low elevations is a statement that discourages or gives reasons why they cannot grow it on low elevations.
From the options, option A is out of point, the question is on Malbec grape and not Bordeaux.
Option B is wrong because it does not specify if Malbec grape is unable to grow in high altitude, it is vague
Option C is also wrong because soils in high altitude having nutrients that will help Malbec grape flourish does not mean soils in low altitude are lacking in these nutrients also.
Option D is correct because the presence of phylloxera in low altitude could be the reason for the restriction of Malbec cultivation to higher altitudes in Argentina.