Final answer:
A horst is an uplifted block of the Earth's crust bounded by two normal faults. In contrast, a graben is a downdropped block bounded by the same type of faults. Horsts indicate areas of crustal extension and influence landscape development.
Step-by-step explanation:
A horst is an uplifted block of the Earth's crust that has moved upward relative to the surrounding terrain. Horsts are typically bounded by two normal faults, where the crust has been extended and the blocks adjacent to the horst (called grabens) have moved downward.
A graben, on the other hand, is a downdropped block that is also bounded by normal faults, making it the contrast to a horst. The distinction between horst and graben structures is significant in the study of geology and geography as they indicate areas of crustal extension and can influence landscape development and geological processes. These structural features can be part of larger geological phenomena such as rift valleys and mountain ranges.
Answer: b. a downdropped block bounded by two reverse faults