The most appropriate sampling method to ensure that the survey results are valid and represent all grade levels equally is:
B. Randomly select a total of 50 students from the entire school (6th, 7th, and 8th grades), ensuring that each grade level is equally represented in the sample.
This method ensures that all grade levels (6th, 7th, and 8th grades) are equally represented in the sample, which will provide a more accurate representation of the homework habits of all students in the school. Options A, C, and D do not adequately account for equal representation across the grades, which could lead to biased results.