EEG stands for electroencephalogram, and it is a medical test that records the electrical activity of the brain. This test involves placing electrodes on the scalp and measuring the electrical signals produced by the brain`s neurons. The recorded signals are then displayed as a series of waveforms, which can help doctors diagnose various neurological conditions and disorders.

Option 1, the heart, is not correct. The electrical activity of the heart is recorded using an electrocardiogram (ECG or EKG), not an EEG.

Option 2, the lungs, is also incorrect. The activity of the lungs is not measured using an EEG. Instead, lung function tests or imaging techniques are used to assess lung health.

Option 4, the muscles, is also incorrect. Electromyography (EMG) is the test used to record the electrical activity of muscles, not an EEG.

Therefore, the correct answer is option 3, the brain. EEG records the activity of the brain by measuring its electrical signals.