The question asks about the specific contaminant that infected plants in the Minamata Valley. Let us analyze the options provided and the relevant environmental impact details:
- Methyl Mercury: The Minamata disease, which affected the Kumamoto Prefecture in Japan (including Minamata Bay), is famously associated with methyl mercury contamination. This occurred due to industrial discharge of methyl mercury into the water bodies, which then bioaccumulated in marine life, and subsequently affected the local population and ecosystem, including plants. Thus, methyl mercury is the correct answer.
- Lead: Although lead is a known environmental pollutant, particularly affecting air and water quality, it is not specifically associated with the Minamata Valley incident. Therefore, it is not the correct option in this context.
- Zinc: Zinc is an essential trace element but can be toxic in large amounts. However, it is not linked to the Minamata Valley incident, and thus, does not pertain to the problem described.
- Methyl Orange: Methyl orange is not a contaminant; rather, it is a pH indicator used in titrations. It is irrelevant to the context of environmental plant infections connected with Minamata.
Based on the explanations above, the contaminant responsible for infecting plants in the Minamata Valley is Methyl Mercury.
Conclusion: The correct answer is Methyl Mercury, as it directly relates to the environmental disaster known as Minamata disease, which involves mercury pollution.