Which of these most directly triggered the main eruption? a. The formation of a bulge on the north slope b. Avalanches of pyroclastic debris c. A massive landslide Mud flows in the Toutle river d. Outgassing of the magma

the answer is C ,

A massive landslide Mud flows in the Toutle river

Explanation:

There had been many earthquakes in the area prior to eruption but this happen to be a massive land slide because a moving magma have over steepened the mountain slopes..this geologic process uses gravity to cause rock, soil, artificial fill or a combination of the three to move down a slope. Slow weathering of rocks as well as soil eruption and volcanic activity also causes landslides