While all the options you provided can present challenges, the most significant struggle for a newly established colony to survive in a new country would likely be:
D. Lack of knowledge of the environment.
Understanding the local environment—including climate, geography, flora and fauna, and available resources—is crucial for survival. A lack of knowledge in these areas can lead to difficulties in securing food, building shelter, and adjusting to new agricultural practices. While getting along with Native Peoples, spreading religion, and finding wealth are also important factors, they can be addressed more effectively if the colonists have a solid understanding of their new environment.