....... Indus River is the longest river in India.

Answer: The

Explanation: Use "the" before names of rivers and many other geographical features (such as mountain ranges, seas, oceans, deserts) and before plural geographical names.

• Examples: The Indus River, the Himalayas, the Pacific Ocean, the Sahara Desert.

Why the other choices are incorrect:

• "An" is an indefinite article used before words that begin with a vowel sound; it is not used before proper names like the name of a river.

• "A" is an indefinite article used before words that begin with a consonant sound; it does not apply to proper geographical names that require the definite article.

• "In" is a preposition, not an article, so it cannot serve as the needed article before the proper noun "Indus River."