Step 1: Understanding the Concept:
Adopter categories are classifications of members of a social system based on their willingness and timing in adopting a new technology or idea.
Step 3: Detailed Explanation:
The primary behavioral variable used to segment individuals into adopter categories is "innovativeness".
Innovativeness is defined as the degree to which an individual is relatively earlier in adopting new ideas than other members of a social system.
Rogers used a normal distribution curve (bell curve) based on innovativeness to divide adopters into five categories: Innovators (5%), Early Adopters (5%), Early Majority (34%), Late Majority (34%), and Laggards (16%).
Compatibility and relative advantage are perceived attributes of an innovation that influence its rate of adoption, but they do not serve as the basis for categorizing the adopters themselves.
The rate of adoption is a system-level measure of the speed at which an innovation is taken up by members of a social system.
Hence, the classification of adopters is based on their individual level of innovativeness.
Step 4: Final Answer:
The basis of adopter categories is Innovativeness, which corresponds to option (C).