Transmountain diversions in the U.S. primarily take water from the wet eastern slopes of the Rockies across to the dry western slopes. The Option A is correct.
Transmountain diversions involve the transfer of water resources from one side of the Rocky Mountains to the other, specifically from the water-rich eastern slopes to the arid western slopes.
This practice has been essential in addressing water scarcity issues in the western United States. But it's important to note that the implementation of these diversions has faced challenges, particularly in the face of increasing water demands and climate change impacts.
As transmountain diversions have been a longstanding strategy for water management, they were not developed in the 1970s to address the Energy Crisis but rather, they have been used for decades to ensure water availability in arid regions.