Final answer:
The major cnidarian clade that jellyfish, the Portuguese man-of-war, and freshwater hydras belong to is Hydrozoa, a class of cnidarians that includes nearly 3,500 species. Hydrozoa cnidarians have both polyp and medusa forms in their life cycle and can form colonies of specialized polyps.
Step-by-step explanation:
The major cnidarian clade that jellyfish, the Portuguese man-of-war, and freshwater hydras belong to is Hydrozoa. Hydrozoa is a class of cnidarians that includes nearly 3,500 species, most of which are marine. Most species in this class have both polyp and medusa forms in their life cycle. Hydrozoa cnidarians can be found in colonies composed of branches of specialized polyps that share a gastrovascular cavity, like the Portuguese Man O'War.