The factor that has the greatest effect on soil formation is (climate, parent material, time, or slope orientation)

The correct answer is - Climate.

All of the mentioned factors contribute a lot to the soil formation, but the climate is the one that has the biggest effect on it. The climate dictates the shape and composition of the environment, so it is able to contribute in the sense that there can be much bigger biodiversity, from it there will be huge amounts of biomass, and if there's enough heat and precipitation, the decomposition of the biomass will be very quick, and the soil will be much richer and of better quality. On the other hand, if the climate is very cold and dry, and the landscape is mostly frozen, than the soil will not be able to properly develop, and it will be very poor in nutrients, low quality, and loose and unstable.