Explanation:

- The sentence in question is: "He was the 5th richest man in (4)______ history of mankind and the richest Indian who ever lived."

- Option 1: "the" is used to refer to something specific or already known, making it a possible choice as it specifies "the history of mankind."

- Option 2: "a" is an indefinite article used for non-specific objects or people; it doesn't fit well in this context.

- Option 3: "one" could imply one among many, but doesn't fit naturally in the context of history.

- Option 4: "an" is another indefinite article, used before vowels, but similarly to "a," it doesn't suit the context of a specific history.

- Option 1: "the" is the most suitable choice since it specifically refers to "the history of mankind," which is a defined and recognized concept.