TT6 should be used in cases of:

The choice indicating that TT6 should be used in cases of Type II DCS (Decompression Sickness) and arterial gas embolism is correct as this therapy is specifically designed to address more severe forms of diving-related illnesses. Type II DCS is characterized by neurological or other systemic symptoms that can manifest after ascent, often linked to bubbles of nitrogen that cause obstruction of blood vessels or damage to tissues.

Arterial gas embolism occurs when gas bubbles enter the arterial system, potentially leading to serious complications such as stroke or tissue ischemia. Using TT6, which involves hyperbaric oxygen treatment, aims to reduce the volume of these bubbles and restore oxygen to affected tissues. As a result, it reflects the necessity for immediate and diligent treatment of these critical conditions which can threaten life and limb.

While other options reflect various diving-related issues, they either pertain to less severe forms of DCS or address cases that do not require the intensity of treatment provided by TT6. Thus, utilizing TT6 in the context of severe conditions highlighted in the correct option is essential for effective management and positive outcomes in undersea rescue scenarios.