Which one of the following is an indirect ecological factor?

Answer: Option A

Soil structure is an indirect ecological factor.
Ecological factor or eco factor is any factor, abiotic or biotic, that influences living organisms.
Abiotic factors include ambient temperature, amount of sunlight, and pH of the water soil in which an organism lives.
Biotic factors would include the availability of food organisms and the presence of biological specificity, competitors, predators and parasites.