Secondary data means data that has already been collected, compiled and published by someone else, so we simply borrow it rather than gather it fresh. Observation, interview and mailed-questionnaire are all ways of collecting primary (first-hand) data directly from respondents. An international agency's published figures, such as …

Step 1 — Primary vs secondary data

Data is classified by who collected it for the purpose at hand:

• Primary data — collected first-hand by the investigator for the current enquiry (observation, personal interview, mailed questionnaire, schedules).
• Secondary data — already collected and published by some other agency, then reused.

Step 2 — Test each option

• (B) Observation, (C) Interview, (D) Mailed questionnaire → all are primary collection techniques (the investigator obtains the data directly).
• (A) An international source like the World Bank publishes ready-made statistics that we merely take over → this is secondary data. …