J’ai vu ____ le monde.

Explanation

The correct answer is tout
The correct sentence would be: "J'ai vu tout le monde." Which translates to "I have seen everyone" or "I have seen the whole world". Here, "tout" is a masculine form of "all" or "whole", and it agrees with the masculine noun "monde" (world).
Note: If the sentence were "J'ai vu toute la ville", then "toute" would be the correct answer, as "ville" is a feminine noun. But in this case, "tout" agrees with the masculine noun "monde".