Hint: The birds constitute a separate class in Kingdom Animalia known as class Aves. These are eukaryotic organisms belonging to phylum Chordata and these organisms have their forelimbs modified into wings for flying.
Complete answer: Struthio is commonly known as the Ostrich. It is the largest bird to exist. An average male adult is about nine feet tall and weighs hundreds of pounds. The female ostrich Is smaller in size when compared to the male birds. The family to which these birds belong is known as Struthionidae. The scientific name of Ostrich is Struthio camelus. Considering the other given options: A. Pavo is commonly known as Peacock. These birds belong to the Pheasant tribe. These birds are not as large compared to Struthio. Thus it is not the appropriate option. C. Pelican is a bird associated with an aquatic environment. Even though these birds have the largest body size compared to other water birds. In the whole class of birds, it is not the largest. It is smaller than the Ostriches. Thus it is not the appropriate option. D. It is not the right option, since D indicates that none of the options given are right. But the option of having Struthio is the right one.
Therefore from the given options, the most appropriate answer is B. That is, the largest bird is Struthio.
Note: Due to their greater body size and weight, Ostriches can’t fly. Apart from Penguins, these are the only flightless birds. The modified forelimbs into wings help them in balancing their body posture.