Answer:
D. stable environments
Explanation:
In the gradual speciation model, changes in species are slow over time that occur in small steps. Change in speciation rate influences two factors that are environmental conditions and population size.
Gradual speciation mostly occur in large populations that move towards evolution and live in a stable environment. The stable environment provides enough time for the evolution to happen and stable habitat for the new species to grow slowly.
Hence, the correct answer is "D. stable environments".