The best-suited population for adapting to an environment that undergoes a sudden change in climate would be:
A population with many phenotypes.
Having a variety of phenotypes increases genetic diversity within the population, which enhances the likelihood that some individuals will possess traits that allow them to survive and thrive under the new environmental conditions. This diversity provides a wider range of responses to changes, which is crucial for adaptation. The other options do not provide the same level of potential for adaptability.