Final answer:
The set of words that contains possessive nouns written correctly is 'stepchildren's room, student's backpack, children's toys, Tom's desk'.
Step-by-step explanation:
The set of words that contains possessive nouns written correctly is 'stepchildren's room, student's backpack, children's toys, Tom's desk'. In possessive nouns, an apostrophe ('s) is used to indicate ownership or possession. When the noun is plural, the apostrophe is placed after the 's'.
Learn more about Possessive nouns