The most important factor in soil formation
1) animal life
2) plant life
3) wind
4) rainfall

The correct answer is - 4) Rainfall.

The rainfall is crucial and the most important factor that contributes to the formation of the soil. The rainfall has such big influence because of few things:

- weathering

The water from the rainfall contributes to the weathering of the rocks, thus making them in little sandy pieces.

- plant life

The plant life needs water to exist, and the rainfall provides that. Further, the plants that die are the biggest factor that helps into the soil formation and fertilization.

- animal life

Once there's rainfall and plants, the animals can exist too, and the animals are very import in both the fertilization of the soil, and fertilization of the plants that also fertilize the soil.

- decomposition

The dead plants and animals decompose much quicker and much more efficiently when there's bigger humidity and water, so the rainfall is essential in this process.