Which volcano's 1991 eruption caused global temperatures to drop?

Mount Pinatubo’s 1991 eruption caused global temperatures to drop. The volcano is on Luzon in the Philippines, where it erupted explosively on 15 June 1991 after centuries of relative quiet.

The eruption reached a Volcanic Explosivity Index of 6 and sent ash and gases roughly 35 kilometers into the atmosphere. Most importantly for climate, sulfur dioxide entered the stratosphere and formed sulfate aerosols that reflected some incoming sunlight back into space.

The resulting cooling reduced global average temperatures by about 0.4°C, with Northern Hemisphere temperatures falling by roughly 0.5–0.6°C. The effect lasted for around a year or more, making Pinatubo one of the clearest modern examples of a volcano influencing global climate.

Mount St. Helens is a common alternative, but its 1980 eruption was much smaller in climatic impact. El Chichón also caused short-term cooling after its 1982 eruption, while Krakatoa’s famous climate effects followed its 1883 eruption. The date in the question identifies Pinatubo.