Adense mix of dry, hot rock fragments and hot gas erupting from volcanic vents at high speed is pyroclastic flow. lahar. fumarole. tephra.

The correct answer is A. pyroclastic flow.

The pyroclastic flow, also known as pyroclastic cloud and pyroclastic density current, represents an very fast-moving chaotic current of hot gas and volcanic matter that is moving away from the volcano itself. It reaches speeds of up to staggering 700 km/h, and the gasses can be as hot as 1,000 C degrees. That combination is fatal to any living organism that will be on its way. It is the deadliest product of the volcanoes on the surface of the ground because unlike the lava flows that most of the time can be escaped from, or the volcanic bombs that are falling on certain places and there's not a big chance of being struck by one, the Pyroclastic flow is destroying everything on its way and can not be escaped from, so when it appears if there's human settlements nearby fatalities are a guarantee.