Question

which is an effect of lower levels of solar radiation striking earth? for example if smog were to block out some of the suns incoming rays.

decreased rates of photosynthesis
increased incidences of skin cancer due to uv radiation
decreased aurora borealis activity due to fewer solar flares
increased phytoplankton (algae) activity

Explanation:

Photosynthesis in plants and algae (like phytoplankton) requires solar energy. Lower solar radiation (due to smog blocking rays) reduces the energy available for photosynthesis, so the rate of photosynthesis decreases. Increased skin cancer would come from more UV (not less), aurora borealis is related to solar flares (not smog - induced radiation blockage), and phytoplankton activity would decrease (not increase) with less solar energy. So the correct effect is decreased rates of photosynthesis.

Answer:

A. decreased rates of photosynthesis