The correct answer is option 1, Flora and Fauna. Flora refers to the plant life in the soil, while fauna refers to the animal life. Both flora and fauna play a significant role in the rate of humus formation in the soil. Flora contributes to humus formation by producing organic matter through photosynthesis and providing carbon compounds to the soil. Fauna, such as earthworms and insects, break down organic matter and speed up the decomposition process, leading to the formation of humus.

Options 2, 3, and 4 also have some influence on the rate of humus formation, but they are not the primary factors. Time, option 2, does play a role as it takes time for organic matter to decompose and transform into humus. Temperature, option 3, affects the activity of microorganisms that decompose organic matter. Parent rock, option 4, can influence the mineral composition of the soil but has a lesser impact on humus formation compared to flora and fauna.