What must occur in order for a hanging valley to form

A hanging valley is the tributary valley that get eroded and deepened by glaciers erosion at a slower rate in valley floor, thus the difference in the two valleys' depth increases over time. This tributary valley composed is more resistant rock which then hangs over the main valley.

Explanation:

A hanging valley is part of the tributary valley that is located above the main valley and is commonly associated with the U shaped valleys, and early vertical sods with a tributary glaciers and is a small volume of ice and this makes a shallower valley and is thus often the waterfalls from the outlet of the upper valley. Since the surface of the glacier is originated at the same height and it appears to be at the main valley. The fjord of Norway is some examples and them from 400 meters deep and 1200 meters wide. Also, thy are a factor of the erosion process and are associated with the site of hanging stream sand valleys.

980 is the correct answer