A green-colored solution would show highest transmittance at:

The highest transmittance of a green-colored solution occurs at the wavelength that corresponds to the color directly opposite on the color wheel, which is red. Green solutions primarily absorb light in the red region of the spectrum; thus, they will allow blue and yellow light to pass through more effectively.

In the visible light spectrum, green appears around 495 – 570 nm but generally peaks around 525 nm. This wavelength is where green solutions will transmit the most light because it closely aligns with the wavelength of green light. If you consider the absorption spectrum, green-colored solutions absorb wavelengths around 600 nm and above, which include the yellow to red portion of the spectrum, hence allowing higher transmittance at 525 nm.

Wavelengths such as 475 nm, which are in the blue range, and 620 nm, which are on the red end, are either not absorbed or absorbed in lesser amounts. Given that the question specifies the wavelength of highest transmittance, 525 nm emerges as the clear answer for a green solution.