The correct answer is option 3. Yellow color light is used as fog light because it has the longest wavelength among all colors except red and orange.

Option 1 states that yellow light is the most scattered by fog, but this is incorrect. In fact, yellow light is less scattered by fog compared to other colors.

Option 2 states that yellow light has the longest wavelength among all colors. While this is true, it does not explain why yellow light is specifically used as a fog light.

Option 4 states that yellow light has the shortest wavelength among all colors not already reserved for other purposes. This is not correct as yellow light actually has a longer wavelength compared to other colors.

The reason why yellow light is used as a fog light is because it has a longer wavelength which helps to cut through dense fog. Red and orange colors are already used for brake lights and stop lights, so yellow is chosen as an alternative due to its similarity to red and orange.