Final answer:
The correct answer is D, where researchers measure characteristics without altering conditions in an observational study.
Step-by-step explanation:
The correct answer is option D. In an observational study, a researcher measures characteristics of interest of a part of a population but does not change existing conditions. Unlike experimental studies where subjects are assigned to treatment groups and exposed to a variable, in observational studies researchers do not intervene or manipulate the environment or conditions. Observational studies are often employed when it is unethical, impractical, or impossible to conduct an experiment, allowing researchers to observe natural phenomena in their existing state to draw conclusions.
An observational study is a type of nonexperimental scientific investigation in which the researcher measures characteristics in a sample but does not attempt to manipulate or control variables of interest. This means that the researcher simply observes and collects data on naturally occurring phenomena without intervening or applying a treatment.
For example, in a study investigating the effects of smoking on lung cancer, researchers would measure and compare lung cancer rates among different groups of people - smokers and non-smokers - without directly controlling or manipulating their smoking habits. By observing and collecting data in this way, researchers can draw conclusions about the relationship between smoking and lung cancer.