Final answer:
The 1986 event that released around 0.15 km³ of gas from Lake Nyos was a gas eruption, specifically involving the release of carbon dioxide gas caused by volcanic activity beneath the lake.
Step-by-step explanation:
In 1986, about 0.15 km³ of gas was released from Lake Nyos over the course of an hour due to a gas eruption, making the correct answer C) Gas eruption.
This tragic event resulted in the release of a large volume of carbon dioxide gas from the depths of the lake, which was saturated with CO₂ due to ongoing volcanic activity beneath it. A controlled degassing vent has since been installed to prevent a recurrence of a similar disaster.