In this food chain, the link "X" needs to be identified. The food chain starts with grass, which is eaten by an insect. The insect, in turn, is eaten by a frog. The frog is then eaten by an eagle.

To find the possible link "X", we need to consider the predator-prey relationship in the food chain. The only option that fits this relationship is option 4 - snake. Snakes are known to feed on frogs, making them a possible link between the frog and the eagle in the food chain.

Therefore, the correct answer is option 4 - snake. It is important to note that snakes are not the only possible link, but in this specific food chain, they are the most likely choice.