Given our understanding of cardiac anatomy, we can confirm that the main structural distinction is that the right atrium has the pectinate muscles, while the left one does not.
Firstly, we must address that there is a wording issue with the question. In reality, it is the right atrium that contains the pectinate muscles. The remaining options either correspond to both atrium, or neither.
The auricle is another word for ear. This is a cavity present on both the right and left atrium which allows them to extend their blood carrying capacity in situations of need. This option cannot be correct for the question since both atria possess one.
As for options b and c, cardiac muscle is present in the entirety of the heart. Cardiac muscle is the muscular cells that make up every wall and cavity of the heart and allow it to complete its unique function. Chordae tendinae, are fibers present in the ventriculus that attach the walls to the tricuspid valve, these are not present in either of the atria.
Therefore, we can confirm that though the wording is off, the intended correct answer is pectinate muscles, given that the right atrium is characterized by their presence, whilst the left atrium contains few, if any, of these muscles.
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