Data are primary to the person or agency that collects them first-hand, but once already-collected data are taken and re-used for a different purpose they become secondary. Here the salary figures were originally collected for all 1000 persons; extracting the women's salaries from that …

Step 1 — Primary vs secondary

• Primary data — collected first-hand by the investigator for the specific enquiry.
• Secondary data — data already collected (by the same or another agency) and then used again for another purpose.

Step 2 — Apply to the situation

The full salary information on 1000 persons is the original collection. When the women's salary data are extracted from this existing set, they are not freshly collected — they are drawn from data that already exist and are put to a new use (studying women's salaries). Re-using already-collected figures makes them secondary for this purpose.

Step 3 — Reject the others

• Primary (A): would require fresh, first-hand collection specifically about women — not the case. …