The correct option is A Kyoto Protocol - 1998
The Kyoto Protocol is affiliated with the United Nations Framework Convention on Climate Change (UNFCCC) regime. This Protocol aims to reduce the level of emission of greenhouse gases (GHG). It was adopted in 1997 in Kyoto (Japan) and came into force in 2005.
Hence option a is the mismatched pair.
The Montreal Protocol was signed in 1987 in Montreal, Canada by various countries to protect the stratospheric ozone. It was aimed at limiting the production and use of ozone depleting substances. It also aimed at helping developing countries to implement alternatives to chlorofluorocarbons.
The United Nations Conference on Environment and Development (UNCED), also known as the Earth Summit was held in Rio de Janeiro, Brazil in 1992. The main aim was to take appropriate measures for the conservation of biodiversity and sustainable utilisation of its benefits.
The latest United Nations Climate Change Conference COP 25 took place under the Presidency of the Government of Chile and with logistical support from the Government of Spain in December 2019.