The correct answer is - a. convergent boundary.
The formation of the longest and highest mountain range in South America can be attributed to the collision of the Nazca tectonic plate and the South American tectonic plate. This two tectonic plates form a convergent boundary. That basically means that they are pushing against each other, and because of the incredible force and pressure, the land starts to lift up gradually in the surrounding are, thus forming a mountain range over time. In this case the result has been the formation of the Andes, that are stretching in a linear manner, in a roughly north-south direction, from the northern part of the continent to its southern part, in close proximity of the Pacific coastline.
