Final answer:
Option D, running laboratory tests with predators, will best help test if the observed habitat preferences were due to competition between species.
Step-by-step explanation:
The addition to the experiment design that will best help test whether the observed habitat preferences were the result of competition between species is option D.
In this option, laboratory tests are conducted in which predators familiar with the dull colored non-poisonous snake are offered poisonous brightly colored individuals and non-poisonous brightly colored individuals to see if the predators show a preference. By comparing the predators' responses, it can be determined if the observed habitat preferences are influenced by competition between species.