To solve this problem, we need to match the items in Column I with the appropriate descriptions in Column II. Let's analyze each item:$\newline \bullet$(a) 6 - 15 pairs of gill slits:$\newline$ - Cyclostomes, such as lampreys and hagfish, are jawless fish that typically have 6 to 15 pairs of gill slits.$\newline$ - This matches with (ii) Cyclostomes.$\newline \bullet$(b) Heterocercal caudal fin:$\newline$ - Chondrichthyes, which include sharks, have a heterocercal caudal fin where the upper lobe is longer than the lower lobe.$\newline$