The function takes a string as input and returns "one" if the input string contains only alphabetic characters and "zero" otherwise.

[CODE]:
```python
def f(x):
    if x.isalpha():
        return "one"
    else:
        return "zero"
```

This code snippet checks if the input string `x` contains only alphabetic characters using the `isalpha()` method. If `isalpha()` returns `True`, it means the string contains only alphabetic characters, so the function returns "one". Otherwise, it returns "zero".