Alluvial fans are composed of material built up from sediment from the foot of the mountains to the valley floor. they are evidence that

Alluvial fans are evidence that the Death Valley is a tectonically active region.

Explanation:

Alluvial fans can be only formed by the slow and gradual depositional action of a river. But for an alluvial to be formed, it is primarily necessary for the river to continuously carry and deposit sediments for a long period of time. The supply of sediments done by the Death Valley in California provides evidence that it is a tectonically active region.