The marked structure on the X-ray is:
(c) Sphenoid sinus(Ref. Scott Brown, 6th ed., 4/3/2)Shown below are X-ray Waters view (occipitomental) with open mouth and X-ray Nose & PNS- lateral view. Note the highest sinus in the lateral view is the frontal sinus.
Nasopharynx
Ethmoidal air cells
Sphenoid sinus
Pituitary gland
Congenital Lesions and Stridor