Final answer:
Different classification schemes result from different opinions on what variables are the most important to compare or contrast.
Step-by-step explanation:
In the context of classification schemes, the most important consideration is what variables to compare or contrast. Different opinions on this matter lead to different classification schemes. Variables are the factors or characteristics that are being studied, analyzed, or compared. They can vary, and their values can change. For example, in the U.S. federal government survey mentioned, the variable being studied is whether high school seniors are planning to attend a four-year college or university in the following year. The answer choices of 'yes' or 'no' represent different values for the variable. Therefore, the correct answer is b. Variables, as they are the most important aspect in determining different classification schemes.