Which of the following examples illustrates a singular possessive noun?

A singular possessive noun indicates ownership by one person or entity and is typically formed by adding an apostrophe and an "s" to the noun. In this case, "the boy's sweater" is the correct choice, as it shows that the sweater belongs to one boy. The apostrophe followed by "s" after "boy" clearly signifies that it is a singular possessive form, indicating ownership.

In contrast, the other examples represent plural possessive nouns or entities that do not denote singular ownership. For instance, "the girls' toys" shows that the toys belong to multiple girls, while "workers' union" indicates that the union is associated with several workers. "Children's books" suggests that the books belong to children collectively, rather than to a single child. Thus, while all are possessive forms, only the example with "the boy's sweater" correctly illustrates a singular possessive noun.