Which of the following plant pigments primarily absorbs light at a wavelength of 420 nm?

Review the absorption spectrum of chlorophyll a: Chlorophyll a has a strong absorption peak in the blue region of the spectrum, around 420 nm. This makes it the primary pigment absorbing light at this wavelength.

Compare with other pigments: Chlorophyll b absorbs light primarily in the blue (around 450 nm) and red regions, carotene absorbs in the blue-green region (around 450-500 nm), and xanthophyll absorbs in the blue region but not as strongly at 420 nm as chlorophyll a.