① Carotenoids and chlorophyll are only distributed on thylakoid membranes of chloroplasts, so ① is correct;

(2) The photosynthesis of plants absorbs the least green light, so it is wrong to irradiate street trees with green light with a wavelength of about 550nm at night for the purpose of beautifying the environment instead of photosynthesis of plants;

③ The light with a wavelength of about 450nm in white light is only a part of light absorbed by plants in photosynthesis, so only using light with this wavelength will reduce the intensity of photosynthesis.

④ The solubility of four photosynthetic pigments in chromatographic solution is carotene, lutein, chlorophyll a and chlorophyll b in descending order, so the solubility of carotenoids in chromatographic solution is greater than chlorophyll, so ④ is correct;

⑤ Magnesium is an important component of chlorophyll. Without magnesium, chlorophyll synthesis is blocked, which directly affects the absorption of light by plants, so ⑤ is correct;

⑥ After the light with the wavelength of 550nm is converted into the light with the wavelength of 670nm, the photoreaction is enhanced, thus promoting the reduction of C3 and reducing the amount of C3, so ⑥ is correct.

So choose C.