Final answer:
The independent variable in the experiment testing the effect of a chemical on an apple orchard is the chemical, as it is the factor being manipulated by the scientist.
Step-by-step explanation:
During an experiment where a scientist is testing the effects of a chemical on an apple orchard, we can identify different types of variables. The variable that can be manipulated by the experimenter is known as the independent variable, which, in this case, is the chemical being applied to the orchard. The independent variable is deliberately changed to observe its effect on the dependent variable, which in this experiment is the number of apples harvested. The orchard that did not receive any chemical serves as the control group to compare the results against the orchard that was sprayed with the chemical.
Given the options provided, the correct answer is D. The chemical.