A psychologist purchases an electroencephalograph (EEG) for use in his research. The psychologist is interested in measuring

A) intelligence.
B) muscle tension.
C) heart rate.
D) brain-wave activity.

An electroencephalograph (EEG) is a device used to measure brain-wave activity, so the best choice is D. This device detects and records the electrical activity of the brain through the electrodes attached to the scalp, providing information about brain function and activity. The EEG is commonly used in clinical psychology and neuroscience research to study different brain states, such as sleep, wakefulness, and activity associated with cognitive or emotional processes. Options A, B, and C are not directly related to the purpose of an EEG.