Which of the following is an example of a variable?

The choice of age as an example of a variable is correct because a variable is defined as any characteristic, number, or quantity that can be measured or counted and that can take on different values. In the case of age, it can vary significantly among different individuals; for example, one person may be 25 years old while another may be 40 years old. This variability makes age a classic example of a variable in research, as it can influence study outcomes and provides a basis for analysis.

In contrast, the other options refer to fixed quantities or attributes rather than variables. The number of participants in a study is a count that remains constant once the study is designed, while the title of the research paper is a static category that does not change. The overall study duration, also a fixed measure once established, does not change during the course of the study. Thus, these choices do not reflect the concept of variability necessary for something to be classified as a variable.