Concerning religion in schools, the court has ruled against all of the following except which one?
a. students prayer on their own throughout the school day
b. starting the school day with a moment of silence for "meditation or voluntary prayer"
c. clergy-led prayers at graduation ceremonies
d. student-led prayers at sporting events

The that the court hasn't ruled against is: B. starting the school day with a moment of silence for "meditation or voluntary prayer" 

Options A,C, and D were ruled against by the court because these activities could be considered as 'forcefully' imposing a religion to another citizen, which considered a violation of the constitution. "Meditation or voluntary prayer" on the other hand, is not considered as a violation because the students could choose not to do it.