Final answer:
The size of a terrestrial world is the most important factor that determines the level of geologic activity.
Step-by-step explanation:
The most important factor that determines the level of geologic activity in a terrestrial world is size. The larger the planet or moon, the more likely it is to retain its internal heat and the more slowly it cools. As a result, larger solid worlds are more likely to exhibit evidence of continuing geological activity on their surfaces.