The correct option is D India is the largest democracy in the world.
An adjective can exist in three forms-positive, comparative and superlative. The positive form is the base form of the adjective. The comparative form expresses a higher degree of some quality. The superlative form expresses the highest degree. For adjectives with three syllables or more, the comparative degree is formed using the word 'more' and the superlative degree, using the word, 'most'.
Option A is correct as largest is the superlative degree of large.
Option B is incorrect as large is the base form
Option C is incorrect as the word 'larging' doesn't exist.
Option D is obviously incorrect as Option A is the correct answer.