Final answer:
For a complete T6 spinal cord injury patient, the latissimus dorsi muscle is key for performing a successful transfer, as it remains functional and can help lift the upper body.
Step-by-step explanation:
In the case of a patient with a complete T6 spinal cord injury, the musculature below the level of the injury is paralyzed, which includes the lower half of the trunk and lower extremities. Therefore, the patient must rely on the muscles that remain functional above the injury site for a successful transfer. The most relevant muscles for this task would include the upper abdominal muscles and muscles of the upper back.
Considering the given options and the anatomy of the muscles involved, the latissimus dorsi is the most likely muscle to be used to lift the buttocks from the mat during a transfer. This muscle is innervated above the T6 level and is capable of providing trunk stability and upper body lifting force that will aid in transfers. The quadratus lumborum, internal obliques, and erector spinae muscles will not be primarily used as they are either paralyzed due to the level of injury or do not provide the necessary movement for the transfer in the context of paralysis at the T6 level.