Explanation:

- A happy body refers to a state of emotional well-being rather than physical health. It doesn't directly relate to fighting off illnesses.

- A healthy body implies physical well-being and fitness, often making it capable of naturally fighting off illnesses. This is the most appropriate option for the blank.

- A flabby body describes having loose and soft tissue, which is unrelated to the body's capability to combat illness.

- A fresh body suggests feeling revitalized or awake, which does not directly connect to the body's ability to fight diseases.