To determine the wavelength or colour at which solar radiation peaks, we need to consider the solar spectrum and the concept of blackbody radiation. The Sun emits radiation across various wavelengths, but the intensity of this radiation is not uniform across the spectrum.

According to Wien's Displacement Law, the wavelength at which the emission of a blackbody spectrum is at its maximum is inversely proportional to its temperature. The formula for Wien's Law is:

\(\lambda_{\text{max}} = \frac{b}{T}\)

where:

• \(\lambda_{\text{max}}\) is the wavelength of maximum emission.
• \(b\) is Wien's constant, approximately \(2.897 \times 10^{-3} \, \text{m K}\).
• \(T\) is the temperature of the blackbody in Kelvin. For the Sun, this is approximately 5778 K.

Calculating the peak wavelength for the Sun:

\(\lambda_{\text{max}} = \frac{2.897 \times 10^{-3} \, \text{m K}}{5778 \, \text{K}} \approx 500 \, \text{nm}\)

The peak wavelength of approximately 500 nm falls within the visible spectrum, specifically in the green region. This means that the Sun's radiation peaks at green light.

Conclusion: The wavelength/colour at which solar radiation peaks is green, hence the correct answer is green.

By understanding the solar radiation and its peak emission, we can conclude that during the daytime, the sunlight appears white because it is a mixture of all colors, even though it peaks in the green wavelength.