Which word represents a singular noun?

The term "Babysitter" clearly exemplifies a singular noun because it denotes a single person who is responsible for caring for children. In grammatical terms, singular nouns refer to one entity, while plural nouns signify multiple entities.

In contrast, the other options denote plural forms or collective groups. "Pencils" refers to more than one pencil, "Countries" encompasses multiple nations, and "Children" is the plural form of child, indicating more than one child. Thus, "Babysitter" is the only option that accurately reflects a singular noun in this context.