Final answer:
Darwin's observation of the similarities between the glyptodont and armadillo suggested B) common ancestry and supported the idea of evolution by natural selection.
Step-by-step explanation:
The similarities between fossil animals and modern animals, such as the glyptodont and armadillo, suggested to Darwin the concept of B) common ancestry. This is because the comparison of characteristics among these species provided evidence that they may have evolved from a common ancestor, as they share many physical traits despite being separated by vast amounts of time. Darwin's observations supported the idea that species change over time and that these changes can lead to the formation of new species, a process known as evolution.