Solution:
Any noun that can be counted is a Countable noun. 'Pen' is the right option because it is the only noun in the sentence, and it can be counted. While 'do' is a verb, 'you' is a pronoun, and 'extra' is an adjective.
"Do you have an extra pen?", He asked.
What is the countable noun in this question?
Any noun that can be counted is a Countable noun. 'Pen' is the right option because it is the only noun in the sentence, and it can be counted. While 'do' is a verb, 'you' is a pronoun, and 'extra' is an adjective.