The correct answer is option 4, infrared radiation. In physics, wavelength refers to the distance between two consecutive peaks or troughs in a wave. Different types of electromagnetic radiation have different wavelengths. Option 1, ultraviolet radiation, has shorter wavelengths than infrared radiation. Option 2, gamma radiation, also has shorter wavelengths than infrared radiation. Option 3, visible radiation, includes the wavelengths of light that are visible to the human eye, which is shorter than infrared radiation. Therefore, option 4, infrared radiation, has the longest wavelength compared to the other options. It is important to note that longer wavelengths correspond to lower frequencies. Infrared radiation is often used to detect heat or thermal energy because it can penetrate through certain materials.