A 4-year-old child presented with a solitary painless mass on the neck. The parents told that the mass would usually swell and become tender during the times of an upper respiratory tract infection. On examination, a smooth, nontender, fluctuant mass along the lower one third of the anteromedial border of the sternocleidomastoid muscle between the muscle and the overlying skin was observed. CECT neck was done. The above structure develops from due to failure of obliteration of: –