United Nations Conference on climate change, COP21 was held in

The answer is B) Paris. COP21 was the 21st Conference of the Parties to the UNFCCC, held at Le Bourget near Paris in December 2015. It produced the Paris Agreement, which entered into force in November 2016 after the required ratification thresholds were met. Its central aim is to hold global temperature rise well below 2 degrees Celsius above pre-industrial levels, with efforts to limit it to 1.5 degrees, through nationally determined contributions.

Why the others are wrong:

• Moscow (A): Russia hosted no COP. Moscow is associated with other summits, not the climate COP series.
• Berlin (C): Germany hosted COP1 in 1995, which launched the Berlin Mandate. That is COP1, not COP21.
• Tokyo (D): Japan hosted COP3 in 1997 in Kyoto, which produced the Kyoto Protocol. Tokyo itself was not the venue.

Core concept: the UNFCCC (1992, Rio Earth Summit) is the parent treaty, and COPs are its annual decision-making meetings. For Mains GS-3 (Environment), the shift from the top-down Kyoto model to bottom-up, voluntary NDCs under Paris is a frequently asked analytical point, including the implications for India's climate commitments and the 2030 targets.

Trap: candidates mix up the COP number with the city. COP1 was Berlin, COP3 was Kyoto, and COP21 was Paris. Memorising the protocol and the venue together, rather than the COP number alone, prevents this error.