World Population Day is observed on

1. A

   11 July

   11 July is the United Nations World Population Day in the usual calendar (linked to 5 billion in 1987). It is listed but is not this paper’s official key. Do not treat the UN date as the stored letter.

2. B

   8 May

   8 May is World Red Cross and Red Crescent Day, not the keyed population date. Mixing humanitarian days is the trap.

3. C

   5 June

   5 June is World Environment Day (Stockholm 1972 memory). It is an environment observance, not the official population day on this row.

4. D

   16 October

   (d) Official key: This paper keys 16 October as World Population Day. Among 11 July, 8 May, 5 June and 16 October, the stored date is 16 October. Honour that official calendar choice.

Summary. Official key is (d). The question asks when World Population Day is observed. The Commission keyed 16 October. 11 July is the familiar UN Population Day; 5 June is Environment Day; 8 May is Red Cross Day; 16 October is also World Food Day in UN lists. Keep the stored date. The official letter is (d).