Oscar has been diagnosed with autism spectrum disorder. when his mother says, "here's your cereal," oscar says, "here's your cereal." what symptom does this illustrate?

This behavior could be an example of echolalia. Echolalia is the repetition of words or phrases uttered by another, but without an understanding of the meaning or appropriate use of the phrase. So, in this case if Oscar is displaying echolalia then he is not saying these words to offer his mother cerial, but is rather merely repeating the sounds.