Answer : 3. "to"
Explanation :

• Option 1 For’ cannot be used because the phrase ‘home for something’ is incorrect.

• Option 2 ‘In’ cannot be chosen because ‘home in something’ does not make any sense in the given context.

• Option 4 ‘within’ cannot be chosen because it does not make the sentence meaningful.

• Option 3 ‘to’ is the best fit. The phrase ‘to be home to something’ means to be the place where that thing is located or, in the case of living things, where they

live, precisely for talking about inhabitants of a given place (be it plants, animals, people, or robots)

• Here, The Mekong is also home to the world’s largest catfish which are the inhabitants of the Mekong.