Final answer:
The left atrium, like the right atrium, has pectinate muscles (option 3) within its auricle. Pectinate muscles are muscular ridges that are not to be confused with trabeculae carneae or papillary muscles, which are found in the ventricles, or with the septomarginal trabecula found only in the right ventricle.
Step-by-step explanation:
Like the right atrium, the left atrium also contains pectinate muscles within its auricle. Pectinate muscles are muscular ridges that are primarily found on the anterior surface of the right atrium and within the auricles of both atria. These muscles are distinct from other structures such as the trabeculae carneae, which are ridges of muscle covered by endocardium located in the ventricles, and the papillary muscles, which extend from the myocardium in the ventricles to which the chordae tendineae attach.
The septomarginal trabecula (moderator band) is found only in the right ventricle and relates specifically to the conduction system of the heart. Therefore, the correct answer is pectinate muscles.