Final answer:
The significant challenge facing colonizers in the year 2100 is rising sea levels, which are a consequence of global warming affecting island size, freshwater availability, and biodiversity. This challenge is aggravated by socio-political resistance to environmental protection measures.
Step-by-step explanation:
In the scenario set in the year 2100, a significant challenge that colonizers might face when establishing a society on a new island untouched by humans is c) Rising sea levels. Global warming is expected to raise ocean levels due to meltwater from glaciers and the expansion of warmer water. This leads to shorelines being inundated, reducing island size, which affects biodiversity and could result in islands disappearing entirely. Additionally, the altered freeze-thaw cycle of glaciers and polar ice could disrupt the availability of freshwater.
This environmental issue is compounded by socio-political challenges, as efforts to combat climate change through reducing emissions and protecting the environment face resistance due to economic and political factors. The question of preserving biodiversity and the natural environment in the face of human population growth poses considerable uncertainties for the future.