In a fault block where one portion moves upward and an adjacent one moves downward, the upper portion is called a horst. Thus, the correct option is C: The horst.
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In a fault block where one portion moves upward and the adjacent portion moves downward, the upper portion is called a 'horst.' The terms 'graben,' 'plateau,' and 'rift' have different geological meanings and do not correctly describe the upward block. Therefore, the best answer is C: The horst. ;