The correct option is A Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
Origin of chordates is from echinoderms. Chordates evolved sometime during the cambrian period. Echinoderms are only invertebrates, whose blastopore forms anus (deuterostomes) and have similarities in embryonic development, type of coelom and larval stages. Hence, option A is correct.