Answer:
Population.
Step-by-step explanation:
Biology is the study of the living organisms. Different field of biology are ecology, evolution, cell biology, molecular biology, biochemistry, biotechnology that favors the living organisms.
The smallest unit to understand and study the evolution and ecology is population. Population is the group of individual of the same species that inhabit in the same area. The students work at the population level in which different species and different populations are studied.
Thus, the answer is population.