According to the safety valve theory, what would occur once the American frontier was closed is that "the population of the country would quickly overwhelm its natural resources."
The safety valve theory implies that a frontier is a new place where people can start over again—a natural place without civilization.
However, the safety valve theory concluded that as frontiers are getting closed, civilization is spreading to the frontiers.
Hence, natural resources such as forests and water would be overwhelmed due to heavy use.
This led to the ratification of the Forest Reserve Act in 1891.
Hence, in this case, it is concluded that the correct answer is option D. "The population of the country would quickly overwhelm its natural resources."
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