Answer:
Cross Sectional Causal Research
Step-by-step explanation:
A cross sectional causal research - can be used to study the relationship between weight of students' school bag & presence of back pain.
Cross sectional study is the study of difference sample units from population at same period of time. Causal research seeks to identify & explain cause effect relationship between variables under consideration (independent causal variable & dependent resultant variable)
In this case, various students are to be examined at same time, so its a cross sectional study. It studies the impact of independent variable 'school bag weight' on dependent variable 'back pain'. So, its a causal research