'World Ozone Day' is celebrated on

1. A

   25th December

   25 December is Christmas, not World Ozone Day. It has no link to the Montreal Protocol calendar. This date is a festival distractor.

2. B

   21st April

   21 April is not the ozone observance (Civil Services Day in India is 21 April). World Ozone Day is in September. April is the wrong month.

3. C

   16th September

   (c) Official key: World Ozone Day (International Day for the Preservation of the Ozone Layer) is celebrated on 16 September, the date the Montreal Protocol was signed in 1987. Among 25 December, 21 April, 16 September and 30 January, the keyed date is 16 September.

4. D

   30th January

   30 January is Martyrs’ Day in India (Gandhi’s assassination), not World Ozone Day. It is a national calendar trap. The ozone date remains 16 September.

Summary. Official key is (c). The UN ozone day is 16 September, marking the 1987 Montreal Protocol. 25 December, 21 April and 30 January are other well-known dates, not this observance. The keyed celebration date is 16 September.