According to Darwin's theory if evolution, which organism are Best able to survive in nature

Organism best adapted to the environment are able to survive in nature!

Explanation:

Suppose there was a green forest but after a volcano eruption all the green colored vegetation is lost and the area turns black. Now some organisms did survive ... you have a black rat, and a white rat. You also have vultures flying in sky looking for prey. Overtime, the organism best adapted to environment can reproduce more and survive in nature. In our example, the black rats are the best adapted because their color matches with the environment so the eagles or vultures would have a hard time spotting them, whereas, a white rat in a black environment can be easily spotted so it has less adaptation to the nature.