The correct answer is option 'D' - Syrinx.

The syrinx is a specialized vocal organ found in birds that produces sound. It is located at the base of the trachea where it splits into the bronchi. Unlike mammals, birds do not have vocal cords or a larynx, so the syrinx is responsible for producing the wide range of sounds that birds are capable of.

The syrinx is a complex structure made up of muscles, membranes, and cartilage. It is unique to birds and allows them to produce a wide variety of sounds, from simple chirps to complex songs. The structure of the syrinx varies among different bird species, which contributes to the diversity of bird vocalizations.

The syrinx is controlled by a set of muscles that can modify its shape and tension. This allows birds to control the pitch, volume, and quality of their vocalizations. Different bird species have different syrinx structures and muscle control, resulting in the wide array of sounds we hear in birdsong.

Birds use their syrinx to communicate with each other for various purposes. They use vocalizations to attract mates, defend territory, and communicate with their offspring. Some species of birds have elaborate songs, while others have simpler calls. The syrinx allows birds to produce these vocalizations with great precision and control.

In conclusion, the syrinx is the vocal organ responsible for producing sound in birds. It is a complex structure located at the base of the trachea and allows birds to produce a wide range of sounds for communication and other purposes.