\( \definecolor{colordef}{RGB}{249,49,84} \definecolor{colorprop}{RGB}{18,102,241} \)
Scenario: A researcher wants to survey 100 students in a high school of 1000. The school population is divided by grade: 9th (250), 10th (300), 11th (250), and 12th (200). The researcher randomly selects 25 students from 9th grade, 30 from 10th, 25 from 11th, and 20 from 12th.
What type of sampling method is being used?