Final answer:
The D-ring on a safety harness should be placed in the middle of the back for optimal force distribution during a fall. Any defects in the thoracic cage would primarily put the heart at risk, as it is one of the vital organs protected by this structure.
Step-by-step explanation:
The D-ring or support point on a safety harness should be placed in the middle of the back, which is option a). This position is crucial because it helps to ensure the most efficient distribution of forces during a fall, reducing the risk of injury. It is vital that safety equipment such as a safety harness is worn correctly to provide the maximum level of safety and protection.
Regarding the thoracic cage, if there were abnormalities or defects, the heart would most likely be at risk, which corresponds with option d). The thoracic cage protects several vital organs, and any deformity could lead to serious health issues, particularly for the internal organs that it encases.