What do proper nouns refer to?

Proper nouns specifically refer to unique names of individual people, places, organizations, and sometimes specific things. They are capitalized to indicate their distinction from common nouns, which represent general categories or groups. For instance, 'Michael' refers to a specific person, and 'Paris' denotes a particular city, both of which exemplify the essence of proper nouns.

In contrast, the other options refer to broader categories. Common objects or things do not have the specificity required of proper nouns; they involve general items, like 'table' or 'book.' Likewise, general ideas or concepts encompass abstract terms that are not tied to distinct entities, and actions performed by nouns describe verbs, which are not represented by proper noun usage. Thus, the focus on unique identifiers in the correct option differentiates proper nouns from other types of nouns.