The word "immune" means protected or resistant to a particular infection, disease, or influence. To find the most appropriate antonym for "immune," we should look for a word that means vulnerable or susceptible to infections, diseases, or other negative influences.

Option 1: "Susceptible" - This is the most appropriate antonym for "immune." It means easily affected by something or susceptible to harm, which is the opposite of being immune.

Option 2: "Fragile" - This word means easily broken or delicate, but it doesn't directly relate to the concept of immunity, so it is not a direct antonym for "immune."

Option 3: "Sick" - While being "sick" may involve a lack of immunity, it doesn't necessarily serve as a direct antonym for "immune" since someone can be susceptible to illness without currently being sick.

Option 4: "Sedentary" - This word means inactive or characterized by sitting, and it is unrelated to the concept of immunity, so it is not a suitable antonym.

Therefore, the most appropriate antonym for "immune" among the options provided is "Susceptible" (Option 1).