Which berry grows on certain cactus varieties in the Southwest US and Mexico?

The prickly pear is a type of cactus native to the Southwest US and Mexico that produces fruit known as cactus fruit or prickly pear fruit. This fruit is recognizable by its vibrant color and distinctive shape. It typically grows at the pads of the prickly pear cactus, making it a unique feature of this plant.

The prickly pear fruit is not only edible but is also rich in nutrients and can be used in various culinary applications, such as jams, syrups, and salads. Its sweet flavor and vibrant color make it a popular choice for both traditional and modern dishes in regions where it is cultivated.

While other options like blackberry and pitaya refer to different types of berries or fruits, they do not specifically grow on cacti found in the Southwest US or Mexico. Thus, the prickly pear stands out as the correct answer within the context of the question.