Which of the following species has the diagnostic characteristic of modified feathers which resemble a beard?

The wild turkey is recognized for its unique and diagnostic characteristic of modified feathers that resemble a beard. This beard is created by a tuft of long, hair-like feathers that protrude from the bird's chest, and it is particularly notable in males. The presence of the beard is often associated with male courtship displays during mating seasons and can be a visual indicator of the bird's health and maturity.

In contrast, peacocks are known for their spectacular tail feathers, or train, which are used in displays but do not have a beard-like structure. Ostriches, being flightless birds, have a different feather structure that serves aerodynamics and insulation rather than ornamentation like that of the wild turkey. Penguins, adapted to aquatic life, possess waterproof feathers that serve to keep them insulated and aid in swimming, without any resemblance to a beard.

The diagnostic feature of the beard in wild turkeys sets them apart from these other species, confirming why this answer is correct.