Which of the following is NOT classified as an extraneous variable?

In the context of research design, extraneous variables are those that can have an effect on the outcome of an experiment but are not the primary variables of interest. They can introduce noise and variability that may obscure the true relationship being studied.

When considering the options, the duration of the experiment is typically a fixed aspect of the methodology and is not usually categorized as an extraneous variable. Instead, it is a controlled variable that is intentionally set by the researcher to ensure consistency in the experiment.

On the other hand, participant motivation, belief in treatment effectiveness, and temperature of the room are all factors that can vary and potentially influence the results of a study. For instance, differences in participant motivation can lead to variations in how effectively they engage with the treatment, while beliefs about treatment can color their responses. Temperature can affect comfort and focus, thus impacting performance.

Therefore, since the duration of the experiment is planned and maintained, it does not classify as an extraneous variable, making it the correct answer to identify.