The correct answer is option 2, the sea horse.

To understand why sea horse has vertebrae, let`s first understand what vertebrae are. Vertebrae are the bones that make up the spinal column or backbone of animals. These bones provide support and protection for the spinal cord.

Option 1, the sea cucumber, does not have vertebrae. Sea cucumbers belong to a group of marine animals called echinoderms, which have a unique skeletal structure called ossicles. These ossicles are not true vertebrae.

Option 3, the sea urchin, also belongs to the echinoderm group and does not possess vertebrae. Sea urchins have a hard, calcified shell composed of plates known as tests.

Option 4, the starfish, is yet another echinoderm that lacks vertebrae. Starfish have a hard, spiny exoskeleton called a test, but they do not have a spinal column.

So, the only animal in the given options that has vertebrae is the sea horse.