Answer:
The correct answer is: It is the tallest building in the world.
Explanation:
One of the ways we can use to build the superlative form is to add the ending -est on the adjective and before the adjective we insert the article the.
e.g. It is the smallest purse I've ever seen.
We can also make the superlative form by inserting most before the adjective.
e.g. She is the most beautiful girl I've ever seen.
