Final answer:
The most common factor in environmental sex determination is temperature, which influences the sex outcome in species with temperature-dependent sex determination.
Step-by-step explanation:
The most common factor in environmental sex determination is d. temperature.
Temperature is a crucial environmental variable that can dictate the sex of certain species through a mechanism known as temperature-dependent sex determination (TSD). In TSD, the sex of an organism, such as in some turtles and reptiles, is determined by the temperature at which the eggs are incubated. A specific temperature range may lead to the development of males, while a different range may result in females.
This demonstrates how environmental factors can influence the expression of the genotype, leading to variations in phenotypes within a population. Unlike other potential environmental stimuli, such as pH, humidity, or ambient light levels, temperature has a more direct and distinguishable impact on sex determination in these species.