Based on the context of changes in the area, the most likely result would be:
A: Permafrost thawing.
This is particularly relevant in regions affected by climate change, where warming temperatures cause permafrost to melt, leading to various environmental changes. However, if the context specifically relates to coastal areas or regions experiencing changes due to climate change, both D: Sea levels increasing and B: Rivers flooding could also be plausible outcomes.
If you are looking for the most immediate and direct impact of climate change on tundra and Arctic regions specifically, A: Permafrost thawing would be the strongest choice.