The correct answer is - She observed a fossil with skeletal structures similar to certain modern organisms.
The scientists was able to come to the conclusion that certain organisms have common ancestor because she was able to study and see the skeletal structures of extinct species and compare them with modern day species, and they had a big resemblance.
The skeletal structure is something that needs a lot of time to change, millions of years. Even then, the basics of the structure will be the same, with the further modifications in place on top of it. It is something that is very useful when comparing extinct species and modern day species, as it can give a nice clue about their connection and common ancestry.
The cats for example, the modern day ones and the first ones that appeared, have almost the same bone structure, where even a trained eye finds it hard to find a difference, and that is a very nice clue about tracing the ancestry.
