African ostriches, South American rheas and Australian emus are all large extant flightless birds - HSC - SSCE Biology - Question 15 - 2011 - Paper 1

The evidence provided by the existence of flightless birds, such as African ostriches, South American rheas, and Australian emus, relates primarily to the concept of biogeography.

Biogeography studies the distribution of species and their ecosystems in geographic space and through geological time. The presence of similar species in different regions, despite being geographically separated, suggests that these birds have a common ancestor, supporting the theory of evolution through natural selection and adaptation to differing environments.

Therefore, the correct answer is (B) biogeography.