1. The problem asks: What kind of data source can be used to collect bivariate data? 2. Bivariate data involves two variables and their relationship. 3. Common data sources include: - a. Questionnaire: collects responses on two variables. - b. Interview: gathers detailed data on two variables. - c. Observation: records two variables by watching subjects. 4. Since all these methods can collect data on two variables, the correct answer is: **d. all**