The question asks about the primary environmental concern associated with permafrost thawing in the Arctic. Let's consider each option and its relevance:

1. Displacement of the indigenous communities: While this is a significant social concern related to permafrost thawing, it is not primarily an environmental issue. The displacement occurs indirectly due to environmental changes but is not the core environmental concern.
2. Release of Methane and Carbon dioxide: This is the correct answer. When permafrost thaws, organic material trapped in it decomposes, releasing greenhouse gases such as methane (CH₄) and carbon dioxide (CO₂). These gases contribute significantly to global warming and climate change, making it the primary environmental concern.
3. Spread of invasive species: Thawing can alter habitats, potentially making them suitable for invasive species. However, while this is a concern, it is not as widespread or critical as the release of greenhouse gases.
4. Increase of UV radiation: This is not directly related to permafrost thawing. UV radiation levels are influenced more by ozone layer conditions than permafrost activity.

The release of methane and carbon dioxide from thawing permafrost represents a positive feedback loop, where increased emissions from thawing lead to higher temperatures, causing more permafrost to thaw. This exacerbates global climate change.

Thus, the primary environmental concern linked to permafrost thawing in the Arctic is the release of Methane and Carbon dioxide.