The cell is the structural unit of life. Unicellular organisms are made up of one cell that performs all the functions and thus, called as a single-celled organism.

The animals who posses a proper notochord are known as vertebrates whereas the organisms who do not have a proper notochord are known as invertebrates.

The vertebral column is a supportive structure derived from the notochord. It is more commonly called the backbone or the spine. Animals that do not possess or develop a vertebral column or backbone are commonly called invertebrates. They include crabs, snails, lobsters, octopuses, sea urchins, worms, etc.

Vertebrates are the animals that possess a dorsal hollow nerve cord, pharyngeal gill slits, and a notochord at some stage of their lives. Human beings are mammals that possess all three of these characteristics and are hence vertebrates.

In nonchordates, heart is present on dorsal side of the body whereas in chordates heart is present on ventral side of the body.

• Invertebrates do not have a notochord, their heart is present on the dorsal side. They are less evolved animals, 
• whereas vertebrates have notochords, their heart is on the ventral side and they are highly evolved animals.

Vertebrates are animals that possess a notochord, a dorsal hollow nerve cord and pharyngeal gill slits at some stage of their lives. They might possess these characteristics during the initial stages of their lives and lose them later on or they might develop these features later in their lives.

Most of the hemichordates are free living but animals like Rhabdopleura are sedentary. They tend to live with solid attachments.

• Vertebrates have a backbone with a spinal cord, whereas invertebrates do not.
  
• The diversity is exceptionally high among the invertebrates compared to vertebrates.
  
• Vertebrates are always bilaterally symmetrical, while invertebrates could show either bilateral or radial symmetry.
  
• Vertebrates are usually large-bodied and move fast compared to invertebrates.
• Vertebrates have a closed blood system, a well-developed brain, either gills or lungs for respiration, and a complex and sophisticated nervous system, whereas those are primitive in invertebrates. Therefore, it concerns that vertebrates have many specializations to extract the best out of the environment compared to invertebrates.
• Despite all these changes, someone could draw a point that invertebrates are more adaptive due to their simplicity, whereas vertebrates do not have good adaptability in comparison because of the specialization. However, I would like to quote a popular quote to finish that in evolution specialization paralyzes and ultra specialization kills the viability of taxons.

• Animals that lay eggs to produce their offspring are known as oviparous animals
• Birds, reptiles and amphibians are examples of oviparous animals.
• Mammals (including human beings) are an example of viviparous animals as they produce offspring by the development of embryo inside their body, eventually leading to live birth.

Animals giving birth to young ones are called viviparous animals. The egg of these animals typically contains less of yolk insufficient to support the development of the zygote into an embryo. Hence, development proceeds inside the body of the female animal and the fully developed embryo is delivered. Oviparous animals lay an egg. Their egg typically has more amount of yolk and can support the development of the zygote into an embryo. Ovovivipary is an in-between scenario in which eggs are retained till embryo is developed and is then delivered.

Tortoise lays eggs so it is Oviparous and not Viviparous.

There are two types of reproduction: Sexual and Asexual.

Ovipary is the phenomenon in which an adult organisms reproduces and lays eggs which later hatch and give rise to young ones.

Animals which give birth to young ones are known as mammals which are vertebrates.

Cow is a placental mammal. Once it gives birth to a calf it grows outside the cow's body.

In viviparous animals, females give birth to young ones. The development of zygote takes place inside the female's body. Females deliver young ones and the chances of survival of offsprings are more. Mammals (except egg-laying mammals), like whale, shark, mouse, humans are examples of viviparous animals.

So, the correct answer is 'Sharks'.