Answer:
The answer is 2. correlational research.
Explanation:
Correlational research intends to understand the relationship between two or more variables in a study. For example, an article named "Listening to classical music might increase your concentration" is likely based on correlational research.
As the passage states, correlational studies do not seek to find a cause-effect relationship. The study in the example might reveal that sleep influences a person's use of a computer (or the other way around), but that does not mean one is the only direct cause for the other.