Final answer:
The contraction of the region can be attributed to tectonic plate subduction, where the Pacific plate is being subducted beneath the North American plate, causing compression and deformation of the Earth's crust.
Step-by-step explanation:
The contraction of the region between the eastern Sierra Nevada in California and the Wasatch Mountain front in central Utah in the last 30 million years can be attributed to c. tectonic plate subduction. Tectonic plate subduction occurs when one tectonic plate is forced beneath another, causing compression and deformation of the Earth's crust. In this case, the Pacific plate is being subducted beneath the North American plate, resulting in the contraction of the region.
As the Pacific plate is being subducted, it exerts compressional forces on the North American plate, causing it to buckle and fold. This compression is responsible for the east-west contraction and the narrowing of the region. Over time, this process has led to the Sierra Nevada mountain range rising and the region between the Sierra Nevada and the Wasatch Mountain front getting narrower.