The outer wall of the oesophagus is not lined by the serous membrane. Instead, it contains
Adventitia externa
Lamina propria
Adventitia interna
Muscularis tunica
As we have learnt in Serosa: It is present only in the region of the alimentary canal within the abdominal cavity. Instead of serosa, the mouth, pharynx, and oesophagus have a dense sheath of collagen fibres called the adventitia.
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So,
The outer wall of the oesophagus is not lined by serous membrane, The outer wall is seen in the form of an irregular coat of yellow elastic dense fibrous connective tissue called adventitia externa or tunica adventitia. Hence, the correct answer is option a.