What anatomical structure separates the left and right nasal cavities?

The nasal septum is the anatomical structure that effectively separates the left and right nasal cavities. It is made up of both bone and cartilage and is located in the midline of the nasal cavity. The bony part of the septum is formed by the perpendicular plate of the ethmoid bone and the vomer bone, while the cartilaginous part is composed of the septal cartilage. This structure plays a crucial role in maintaining the shape of the nasal cavity and ensuring that airflow is evenly distributed between the two sides.

The other options do not fulfill this role. The nasal conchae are bony structures within the nasal cavity that help to filter and warm the air but do not divide the nasal cavities. The zygomatic bone is involved in the formation of the cheeks and lateral walls of the orbits and has no role in separating the nasal passages. The sphenoid bone is located deeper in the skull and contributes to the cranial cavity as well as areas around the nasal cavity but does not act as a separator between the left and right nasal sides.