Final answer:
The generalization that can be made from the results of the observational study is that later high school start times are associated with higher average first period year-end grades.
Step-by-step explanation:
The generalization that can be made from the results of the observational study is B) Later high school start times are associated with higher average first period year-end grades.
The study stated that a high school increased the length of the school day from 6.5 to 7.5 hours. By the end of one year, student performance on statewide tests had increased by 10 percentage points over the previous year. This suggests that the longer school day contributed to the improvement in student achievement.
However, the study also mentioned that there could be other factors at play, such as self-selection of motivated students who were willing to sign the contract and put forth their best effort. Therefore, while the results suggest a positive association between later high school start times and higher grades, it would be important to consider other potential variables that may have influenced the outcome.