Final answer:
Biodiversity describes the variety of life, including species, genetic, and ecosystem diversity. The correct answer is A) The number and variety of species within an ecosystem, reflecting both the number of species and their relative abundances.
Step-by-step explanation:
The term biodiversity describes the variety of life on Earth, encompassing several different aspects: species diversity, genetic diversity, and ecosystem diversity. The correct answer to the student's question, 'Which of the following best describes the word 'biodiversity'?' is A) The number and variety of species within an ecosystem. Biodiversity can be measured at multiple organizational levels, traditionally evaluating both species number and relative abundance. Due to current high rates of species extinction, which are estimated to be 500-1000 times the normal rate, there's a significant decline in biodiversity anticipated in the approaching century with vast implications for ecosystems, including those rich in biodiversity such as the tropical rainforests.