Animals and plants that live in russia’s tundra region must adapt to extreme cold and dry conditions are found mainly on the forest floor are larger than plants and animals in other ecosystems must adapt to hot summers with abundant rainfall

The correct answer is  - must adapt to extreme cold and dry conditions.

In the tundra region, the plants are small, they are low to the ground, leaned sideways and attached to a rock if possible. 
Animals have thick long furs with great isolation, usually accumulate fat, wide hairy paws, small rounded ears...
These adaptations in both the animals and plants are because of the extreme conditions in this region, winter that lasts for most of the year, extreme cold, very strong cold winds, low humidity, low precipitation, life on ice.