Identify the error: 'She has a twenty-year old son.' MCQ with Answer and Explanation

Correct form is 'twenty-year-old' with hyphens. Without the final hyphen, it might still be accepted, but usually 'a twenty-year-old son'. So 'twenty-year-old' is hyphenated. The option has 'twenty-year old', missing a hyphen. So error at 'twenty-year' if not fully hyphenated. I'll say no error in colloquial writing, but standard exam expects 'a twenty-year-old son'. So error is that it should be 'twenty-year-old'. So 'twenty-year' is incomplete. I'll mark B.