This argument relies on faulty causal reasoning based on the idea that because climate fluctuation occurred at the same time as the fall of the Roman Empire that means it was probably a major cause of the fall of the Roman Empire. The author provides additional support by stating that this could be true because fluctuating weather leads to an unstable food supply and thus an unstable empire.
To strengthen this causal argument we have several options:
1) show without cause then no effect, show additional instances of the cause creating the same effect, give evidence that the relationship is not reversed, provide additional statistical support, and rule out an outside alternative cause.
A) is incorrect because it weakens the connection between climate -> food shortage -> collapse by showing political failures could be an alternative explanaton.
B) is incorrect because it weakens the argument by stating even with the cause there is no effect.
C) is incorrect because it weakens the argument by providing an alternative cause of food shortages.
D) is correct because it shows an additional instance where there is both the cause and the effect.
E) is incorrect because it is out of scope by talking about Europe instead of the Roman Empire alone.