While all the options listed can pose challenges for a newly established colony, a major struggle that stands out is:
D. Lack of knowledge of the environment
Understanding the local environment is crucial for survival, including knowing what resources are available (such as food, water, and materials for shelter), recognizing potentially dangerous wildlife, and adapting to climate conditions. A lack of knowledge in these areas can lead to significant hardships for the colony.
Getting along with native peoples (A) can also be critical since cooperation could aid in survival, but the immediate need for environmental awareness might take precedence.