Final answer:
The sphenoid sinus is the sinus positioned directly behind the nose at the center of the skull and is the most posterior of the paranasal sinuses.
Step-by-step explanation:
The paranasal sinus located directly behind the nose at the center of the skull is the sphenoid sinus. This sinus is the most posterior paranasal sinus and is situated within the body of the sphenoid bone, just anterior and inferior to the sella turcica.
In contrast, the frontal sinus is the most anterior and located above the eyebrows, the maxillary sinuses are the largest and located below the orbits, and the ethmoid sinus consists of multiple small spaces called ethmoid air cells, located on both sides of the ethmoid bone between the upper nasal cavity and medial orbit.