To solve this problem, we need to identify the true statements about the phylum Chordata. Let's analyze the characteristics of Chordata:$\newline \bullet$Chordates are animals that possess a notochord at some stage in their life cycle.$\newline \bullet$They also have a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail at some point in their development.$\newline \bullet$Chordates include subphyla such as Vertebrata (vertebrates), Cephalochordata (lancelets), and Urochordata (tunicates).$\newline$Now, let's evaluate the options:$\newline \bullet$Option (a): All chordates possess a notochord at some stage in their life cycle. This is true.$\newline \bullet$Option (b): Chordates have a dorsal hollow nerve cord. This is true.$\newline \bullet$Option (c): Chordates have pharyngeal slits at some stage of their life cycle. This is true.$\newline \bullet$Option (d): All chordates are vertebrates. This is false, as not all chordates develop a vertebral column.$\newline$Based on the analysis, the true statements are (b) and (c).$\newline$Therefore, the correct option is Option 3: (b) and (c).