United Nations Conference on climate change, COP21 was held in

1. A

   Moscow

   Moscow has hosted other diplomatic meetings, not UNFCCC COP21. Russia is not the keyed COP21 city.

2. B

   Paris

   (b) Official key: COP21, the United Nations climate conference, was held in Paris in 2015 and produced the Paris Agreement. Among Moscow, Paris, Berlin and Tokyo, Paris is the official venue.

3. C

   Berlin

   Berlin is associated with earlier climate diplomacy (including a 1990s COP) but not COP21. Do not pick a German capital for 2015.

4. D

   Tokyo

   Tokyo is not the COP21 host. Japan has hosted other environmental meetings; 2015 climate COP was Paris.

Summary. Official key is (b). UN climate COP21 was Paris 2015, source of the Paris Agreement. Moscow, Berlin and Tokyo are wrong capitals. Keep Paris. The official letter is (b).