Final answer:
The biologist used quadrats to study the population of red-backed salamanders and estimate their density and size.
Step-by-step explanation:
The biologist in this study used quadrats to sample the population of eastern red-backed salamanders within a 100 m by 100 m field. Quadrats are square areas used to count the number of individuals within their boundaries.
Based on the data provided, the population density and size estimates for the salamanders are as follows:
a) Population density: 0.03 salamanders/m², Population size: 18 salamanders
b) Population density: 0.09 salamanders/m², Population size: 27 salamanders
c) Population density: 0.15 salamanders/m², Population size: 20 salamanders
d) Population density: 0.20 salamanders/m², Population size: 30 salamanders