Final answer:
The common landform found in regions 5, 7, 10, and 12 is plains. Option d is the answer.
Step-by-step explanation:
The single type of landform found in regions 5, 7, 10, and 12 is plains. Specifically, these regions are known for the following landforms: Region 5 is part of the Cascade-Sierra Mountains, which are characterized by mountainous landforms. Region 7 encompasses the Coastal Plain, which is not mountainous. Region 10 is the Great Plains, and Region 12 is Lower California, which are both distinguished by their plains landforms. From this information, we can deduce that the common landform across the specified regions is plains (D), considering that regions 7, 10, and 12 are indeed synonymous with various types of plains, as opposed to mountainous or plateau regions.