Foie gras refers to the fattened ___ of a duck or goose.

Foie gras is a luxurious delicacy made from the liver of a duck or goose that has been specially fattened through a process called gavage, where the birds are fed a high-starch diet that causes their livers to enlarge significantly. This process results in a rich, buttery, and smooth end product that is highly sought after in gourmet cuisine. The liver is uniquely rich in flavor and has a distinctive texture that sets it apart from other parts of the bird, making it the focal point of various high-end dishes.

The other options, such as breast, leg, and thigh, refer to different cuts of meat from the bird that do not offer the same culinary qualities or traditions associated with foie gras. This specialization is what makes foie gras a unique and prized ingredient in the culinary world.