As you move down a group, atomic radius increases
They have chloroplasts. They do not possess mobility because they cannot just stand up and walk to the nearest river. Unicellular organisms are very small, and most organisms are multi-cellular, including plants. Plants, animals, and fungi are eukaryotic, but only bacteria is prokaryotic. So therefore, choice 3 is correct.
phytoplankton falls on the first trophic level which is are "primary producers", it is because even smallest to smallest creatures like zooplankton feed on phytoplankton.