Final answer:
A scatter plot with a linear trend and a positive correlation coefficient would support the teacher's observation that students who study longer for a test generally earn higher scores.
Step-by-step explanation:
The scatter plot that supports the teacher's observation that students who study longer for a test generally earn higher scores is one where the data points are positively correlated and form an upward sloping line. In other words, as the study time increases, the test scores also increase. This indicates a linear relationship between the variables. A scatter plot with a linear trend and a positive correlation coefficient would be a good representation of this observation.