Correct Option (D)
The passage highlights the severe consequences of Ireland's almost complete dependence on a single variety of potato, the 'Lumper', as its staple food. This genetic homogeneity rendered the entire food source highly vulnerable to a specific pathogen, the 'Phytophthora infestans' fungus. The subsequent destruction of the crop led to widespread famine. Therefore, the critical message is that relying on a homogeneous food source is undesirable due to the inherent risks of susceptibility to diseases or other environmental factors, which can have catastrophic societal impacts.
Incorrect Options:
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Option (A) is incorrect because the passage, while mentioning the potato's suitability for Ireland's climate, primarily focuses on the dangers arising from the lack of genetic diversity within the crop, rather than the initial climatic suitability for introducing foreign plants.
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Option (B) is incorrect because the passage explicitly states that the potato became a "reliable backup to cereal crops" and the Irish were "almost wholly dependent on the potato as their staple food." The issue was the reliance on a single variety, not the general inability of tuber crops to serve as staple foods.
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Option (C) is incorrect because the passage's central message is not about the general unpreventability of fungal infections. Instead, it underscores how the genetic frailty of a homogeneous food source, when exposed to a fungus, can lead to widespread devastation, irrespective of the fungus's general controllability.