Almost the entire system of an embryo is made of cartilage, whereas that of an adult is mostly made of bones.
An embryo is a multicellular diploid eukaryote in an early stage of embryogenesis, or development. In general, in organisms that reproduce sexually, an embryo develops from a zygote, the single cell resulting from the fertilization of the female egg cell by the male sperm cell.
Biologically, an adult is a human being or other organism that has reached sexual maturity.
The correct answer between all the choices given is the first choice or letter A. I am hoping that this answer has satisfied your query and it will be able to help you in your endeavor, and if you would like, feel free to ask another question.