Final answer:
The correct possessive form for the noun 'shirt' is 'man's'.
Step-by-step explanation:
The correct possessive form for the underlined noun 'shirt' in the sentence 'The man shirt was stained' is man's, indicated by the apostrophe followed by the letter 's'. This shows that the shirt belongs to the man. The plural form would be 'men's' if multiple men owned the shirts.
Learn more about Possessive form of nouns in English