Final answer:
A hypothesis is a proposed explanation for a phenomenon, which can be tested. Statement A) "Plants grow best in the presence of light" is a hypothesis and can be tested, whereas the other statements are established facts or theories.
Step-by-step explanation:
A hypothesis is a proposed explanation for a phenomenon, often based on limited evidence and the starting point for further investigation. Among the statements provided, A) "Plants grow best in the presence of light" is a hypothesis. It suggests a possible explanation for how plants might grow, which can be tested through experimentation. On the other hand, B) "All living things are composed of cells." C) "Matter is composed of atoms." and D) "Many human diseases are caused by microorganisms." are statements of established scientific theories or facts, not hypotheses.