Final answer:
Observational studies would be used for questions 1-3, while an experiment would be used for question 4.
Step-by-step explanation:
1) The answer to the first question is a) Observational study. An observational study is used when it is not possible or ethical to conduct an experiment. In this case, researchers would observe and collect data on the proportion of community college students who successfully complete a transfer-level English course. They would not manipulate any variables but instead take measurements of naturally occurring phenomena in this context.
2) The answer to the second question is a) Observational study. Similarly, an observational study would be used to explore whether non-elite soccer players are less likely to develop arthritis of the hip or knee compared to elite soccer players. Researchers would observe and collect data on the occurrence of arthritis in both groups without manipulating any variables.
3) The answer to the third question is a) Observational study. Shifting flora and fauna to new habitats and its impact on the extinction of species would be examined through an observational study, as manipulating this process would not be feasible or ethical. Researchers would observe and collect data on the relationship between shifting habitats and species extinction.
4) The answer to the fourth question is b) Experiment. To determine if the consumption of simple carbohydrates increases the oxidation of LDL cholesterol, researchers would need to conduct a controlled experiment. They would manipulate the independent variable (consumption of simple carbohydrates) and measure the dependent variable (oxidation of LDL cholesterol) to establish a cause-and-effect relationship between the two variables.