Based on the context of Nixon's decision to stop selling soy to Japan, the most accurate interpretation would be:
Nixon’s decision to stop selling soy to Japan caused Japan to invest in agriculture in its own country.
This response reflects the idea that by halting exports, Japan would have had to seek alternative solutions to meet its soy needs, potentially leading to increased investments in domestic agricultural production. The other options either suggest environmental impacts that may not be directly related or imply a negative economic consequence (like a shortage) for Japan, while the last option suggests a positive response from Japan to adapt to the situation.