What boundary is at the Pacific plate sliding under the South American plate, creating the Andes Mountains? (answer fast pls)

The plate boundary is a convergent one where a subduction zone is created.

Explanation:

The Andes are the longest and highest mountain chain in South America, and among the largest ones in the world as well. It is a mountain chain created because of the collision between two tectonic plates and the process is still not over, so the Andes are still growing in size and elevation.

The plates that collide are the Nazca Plate (not the Pacific as it is stated in the question) and the South American plate. The Nazca Plate is a smaller plate and it is mostly made up of oceanic crust, while the South American plate is much larger and is mostly made up of continental crust. The lower and denser Nazca plate moves below the South American plate, thus it subducts, and as it does so, the crust near the plate boundary is pushed up, creating a mountain chain, in this case, that being the Andes.