Final answer:
The variable of interest is the two different class test results, which reflects the effectiveness of the teaching methods by showing students' performance in exams.
Step-by-step explanation:
The variable of interest in the experiment comparing the effectiveness of two teaching methods in two sections of Algebra I for 9th-grade students is D. the two different class test results. This variable represents the outcomes measured to determine if one teaching method is more effective than the other. In educational research, it is crucial to evaluate the effectiveness of teaching methods by assessing students' performances, which is typically done through their results on tests or assessments tailored to the material taught.
In the given context, the teaching methods are the independent variables, while the class test results are the dependent variables that are affected by the teaching methods applied.