Answer: A. Red.
Answer Analysis: To determine which color LED light has the longest wavelength, we need to understand the basic properties of light and the relationship between color and wavelength.
First, the color of light is closely related to its wavelength. In the visible spectrum, red light has the longest wavelength, approximately between 620-750 nanometers, while violet light has the shortest wavelength, approximately between 380-450 nanometers. Green and blue light have wavelengths between these two extremes.
Now, let's analyze the options one by one:
A. Red - As mentioned earlier, red light has the longest wavelength in the visible spectrum.
B. Green - Green light has a shorter wavelength than red light, but longer than blue light.
C. Blue - Blue light has a relatively short wavelength in the visible spectrum.
D. Purple - Purple light has the shortest wavelength in the visible spectrum.
Based on these analyses, we can clearly see that the wavelength of red light is the longest of the four options.
Therefore, the answer is A. Red LED light has the longest wavelength.