Select the taxon mentioned that represent both marine and freshwater species
A. Echinoderms
B. Ctenophora
C. Cephalochordate
D. Cnidaria

Hint: Since all of the given taxon live in the aquatic habitat so you can look for the other specific character of the taxon species. Freshwater species are generally sessile so that they do not get flow away with the water.

Complete answer:
From the classification, it is clearly understood that all the species have aquatic habitat. The species living In the marine habitat are generally free living and they float inside the water in order to capture their food.
If we first talk about the taxon cnidaria, then the common characteristics of Cnidaria are that they are free living species and the habitat is exclusively marine water and they are not found in the freshwater. They have a comb like structure on their head which helps them in the locomotion. They have lasso cells on their tentacles which help them in capturing their prey.
The echinoderms are also marine organisms having a water vascular system. Since we know that this is a marine water species, we would like to know about the locomotion. These species have a tube-like foot which makes them free living and thus they can catch their prey easily. Also, the body is covered with spines.
Cephalochordates are the only species among the given options which have notochord. They also are marine water organisms and since they have a notochord then they can also move from one place to another. They have cilia and with the help of it they catch their prey. Now the last option we have is the Cnidaria. The species of Cnidaria are known to be living in both fresh water as well as marine water ecosystems. Also, the species of the cnidaria are known to be both sessile and free living indicating that they can live in freshwater and marine water.

Therefore, the correct option is the option D.

Note:
The members of the Cnidaria species can be living solely alone or they can also live in the colonies of other cnidarians. The reproduction in the Cnidaria can be by budding which gives solitary species and they can also fuse gametes leading to the formation of colonies.