Which bursa protects the supraspinatus muscle tendon?

The subacromial bursa is the correct choice as it is specifically located beneath the acromion of the scapula and above the supraspinatus tendon. This bursa plays a critical role in reducing friction between the supraspinatus tendon and the overlying bony structures as the shoulder moves. Its position makes it particularly important for shoulder motion, especially during activities that require arm elevation, as it protects the tendon from irritation and damage.

In contrast, while the subscapular bursa is associated with the subscapularis muscle and provides cushioning in that region, it does not directly protect the supraspinatus tendon. The transverse bursa is not commonly referenced in anatomical discussions regarding the shoulder; instead, it typically refers to structures associated with other joints. The posterior bursa is also less frequently mentioned and lacks the specific relevance to the supraspinatus tendon that the subacromial bursa has. Thus, the subacromial bursa's anatomical positioning and function make it the key protective structure for the supraspinatus muscle tendon.