Here's why:
* Common nouns refer to general things, not specific ones. "Pen" refers to any writing instrument of that type, not a specific pen.
* Proper nouns are specific names of people, places, or things (like "Bic" or "The Fountain Pen").
* Abstract nouns refer to concepts or ideas (like "writing" or "creativity").
* Collective nouns refer to groups of people or things (like "flock" or "team").
Since "pen" refers to a general type of object, it's a common noun.