Final answer:
The 1980 Mount St. Helens eruption was caused by the failure of a bulge on the north flank that resulted from earthquake-triggered instability, leading to a landslide and a dramatic drop in gaseous magma pressure and resulting in primary ecological succession.
Step-by-step explanation:
The stupendous blast at Mount St. Helens in 1980 was caused by a series of events that led to one of the most notorious volcanic eruptions in the United States' history. Initially, a series of earthquakes beneath the mountain indicated the reawakening of the volcano, which resulted in the formation and growth of a bulge on the north flank due to rising magma. This magma exerted pressure on the volcanic structure, changing the volcano's shape.
On May 18, an earthquake triggered the failure of the bulge, leading to a landslide of the mountain's northern slope. This landslide dramatically reduced the pressure on the gaseous magma, resulting in a catastrophic explosive eruption. The effect of this eruption decimated the surrounding forests and replaced them with volcanic debris, shaping a new landscape that lacked prior ecosystem remnants like seeds in the soil. This process set the stage for primary succession, a process by which an environment is colonized for the first time by living organisms.
As for the options presented in the original student question, it is revealed that the landslide and the subsequent pressure drop on the magma were the primary causes leading to the explosive eruption, so options A (failure of the bulge) and D (beginning of a landslide) collectively describe the trigger of the event. However, for the specific item in question no. 10, regrowth of vegetation without the remnants of the previous ecosystem would best be described by D (volcanic eruptions), since it is the eruption that directly leads to the type of landscape overhaul where primary succession can occur.