Answer:
There are mountain ranges at passive margins because they are remnants of geological activity in the past.
Step-by-step explanation:
The mountain ranges form when two plates collide, thus there is a convergent plate boundary, where the crust rises up, or there is intense volcanic activity. The passive margins though are far away from the plate boundaries but they still have mountain ranges. While this may come weird and as a surprise, it is in fact very simple to explain.
The mountain ranges at the passive margins are simply remnants of very large and high mountain ranges that have formed in the distant past. Good examples of mountain ranges of this type are the Appalachians and Guyana Mountains. These mountain ranges formed hundreds of millions of years ago, and they were very high mountain ranges. Even with the erosion gradually making them smaller over such a long period of time, they still persist and are dominant in their respective areas.