Which of the following sentences uses the word "subtle" correctly?

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.

The american colonists strongly opposed the tax laws imposed by the british government. the american colonists were angered by the presence of british soldiers who had found employment in america