Explanation:

- Gizzard: This is the correct option. Red worms use the gizzard to grind their food. It's a muscular structure that helps break down food particles through mechanical digestion.

- Esophagus: The esophagus is a tube that connects the mouth to the rest of the digestive system. It mainly transports food and does not grind.

- Intestine: The intestine's primary role is to absorb nutrients. It's not involved in grinding food.

- Crop: The crop temporarily stores food before it moves to the gizzard. It’s a storage space rather than a grinding organ.