Answer:
The correct answer is Amphibians.
Explanation:
An animal is considered as vertebrate when they have vertebral column that is extended from head to tail which protects the spinal cord in the body and also provide structural support. Those animals which do not have this vertebral column are called invertebrate.
Vertebrates comes under the sub phylum vertebrata which includes fish, mammals, birds, mammals and amphibians. Sub phylum vertebrata comes under the phylum chordata which includes both vertebrate and invertebrate.
So in this question only amphibians have vertebral column and not other so amphibians are the only vertebrates.
