Explanation:

- Option 1: Style of life

- Style of life refers to the behaviors, interests, opinions, and activities that individuals express, influenced by cultural and social norms.

- Warner used lifestyle indicators to determine social class in Yankee City.

- This is the basis Warner used for class differentiation in his study, making it the correct answer.

- Option 2: Occupation

- Occupation involves the work or profession in which a person is engaged.

- While important in assessing social class, Warner focused more on lifestyle.

- Option 3: Educational background

- Refers to the formal education and training an individual has received.

- Although relevant to class analysis, it was not the primary focus in Warner’s stratification study.

- Option 4: Income

- Income indicates the financial earnings of individuals or families.

- Though a significant factor in social stratification, it was not the core measure Warner used.