The actual sentence talks about
the three heaviest animals
of the world. A. The Blue Whale, African Elephant, and Indian Elephant are
three heaviest animals in the world
The animals are not just (any) three heaviest animals. We need "the" to imply that they are the (top) three heaviest animals. B. The three heaviest animals are the Blue Whale, African Elephant, and Indian Elephant
The actual sentence has scoped the statement with "in the world". Without the scope, this choice changes the meaning.C. The Blue Whale, African Elephant, and Indian Elephant are
the three heaviest animals
in the worldLooks correct.D. The three heaviest animals in the world
is the Blue Whale, African Elephant, and Indian Elephant
S-V agreement issue. "animals" is plural subject.E. The Blue Whale, African Elephant, and Indian Elephant are the three heaviest animals
The actual sentence has scoped the statement with "in the world". Without the scope, this choice changes the meaning.Choice C fixes all the issue.
Hence,
C