Choose the best fit for the blank from the given options: India is one of…

Concept: The definite article "the" is used before a superlative adjective (the tallest, the best, the most populous) because a superlative narrows a group down to one specific top-ranking member or subset — this specificity requires a definite article, regardless of whether the noun that follows is singular or plural.

Application: In this sentence, "most populous" is a superlative modifying "countries." The phrase refers to a specific, uniquely identified subset of countries — those with the highest population — so the noun phrase is definite and requires "the": "India is one of the most populous countries in the world."

Contrast with the other options:

• "a" is an indefinite article used for a single, non-specific item ("a country"); it cannot introduce a definite superlative group.

• "an" carries the same indefinite-article problem as "a," and is additionally reserved for words with a vowel sound, while "most" begins with a consonant sound.

• Leaving the blank empty ("No article") treats "most populous countries" as a general, unspecified plural, but the superlative makes it a specific, definite set that needs an article.