The function takes an input string and returns the number of unique characters in the input string in word form. For example, if the input string has 3 unique characters, the function returns "three".

[CODE]: 
```python
def f(x):
    unique_chars = len(set(x))
    number_words = {0: "zero", 1: "one", 2: "two", 3: "three", 4: "four",
                    5: "five", 6: "six", 7: "seven", 8: "eight", 9: "nine", 10: "ten"}
    return number_words[unique_chars]
```