Answer:
Fun question!
The answer would be A. I know this from my environmental science class, but it can be deduced these ways:
Root Words
The Latin root word Bio means life. Bio+logos=biology or, the study of life. Therefore, it can be reasoned that bio+mass will be mass that comes from life, or living things.
Context Clues
Context Clues are subtle hints that authors drop readers that help the reader to infer things about the text. In the above text, the author literally says this: "biomass, or plant material", AKA, mass that comes from life, and bam! you have your answer.
hope this helps!