Are these your options?
1- The song is usually played on a CD player because it is subtle.
2- Most films have ratings that are meant to be extremely subtle.
3- There was a subtle smile on her face that said she had a secret.
4-Many people love to subtle their food when their are hungry
If so, then the correct answer is 3. There was a subtle smile on her face that said she had a secret.
Subtle means delicate, discreet, fine, not easily perceptible, so this sentence definitely fits the definition, whereas the other examples do not make much sense.