Ownership for singular nouns is shown by adding -'s to the noun. For example, if you take the noun "cat," you would indicate ownership by writing "the cat's toy."
Here’s a breakdown of the options you listed:
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Add -s to the noun: This is not correct for indicating ownership.
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Add -'s to the noun: This is the correct answer for singular nouns (e.g., "the girl's book").
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Add -es to the noun: This is not correct for ownership.
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Add an apostrophe if the noun already ends in -s: This is correct for plural nouns ending in -s (e.g., "the dogs' park"), but for singular nouns that end in -s, you generally add -'s (e.g., "James's book").
So, for singular nouns, you should add -'s to indicate ownership.