Statement (b) is correct: In subphylum Vertebrata, the notochord is present during the embryonic period and is replaced by a cartilaginous or bony vertebral column in the adult . Statement (c) is correct: The central nervous system in chordates is dorsal, hollow, and single . Statement (a) is incorrect: In Urochordata, the notochord is present only in the larval tail, whereas in Cephalochordata, it extends from head to tail . Statement (d) is incorrect: Phylum Chordata is divided into three subphyla: Urochordata, Cephalochordata, and Vertebrata; Hemichordata is a separate phylum .