Final answer:
Mount St. Helens is a volcano associated with a convergent lithospheric plate boundary, where subduction leads to volcanic activity.
Step-by-step explanation:
Mount St. Helens is a volcano that is associated with a convergent boundary.
This type of lithospheric plate boundary involves the movement of two tectonic plates towards each other, often resulting in one plate being forced under the other into the mantle, a process known as subduction.
The water released from the subducting plate lowers the melting temperature of the mantle, leading to magma formation and volcanic activity.
Therefore, the correct answer to which type of lithospheric plate boundary Mount St. Helens is associated with is A) convergent boundary.