Option 2 states that green light is the least effective in photosynthesis. Photosynthesis is a process in which plants convert light energy into chemical energy. The process occurs in specialized organelles called chloroplasts, which contain pigments like chlorophyll that absorb light energy.

Option 1, blue light, is incorrect because blue light is actually one of the most effective wavelengths for photosynthesis. It is absorbed by chlorophyll and helps drive the process.

Option 3, red light, is also incorrect. Red light is another wavelength that is efficiently absorbed by chlorophyll and is important for photosynthesis.

Option 4, sunlight, is a broad spectrum of light that contains a range of wavelengths, including blue and red light. Sunlight is necessary for photosynthesis to occur, so it cannot be the least effective in photosynthesis.

Therefore, the correct answer is option 2.