I borrowed _ pencil from your pile of pencils and pens.

Explanation

Explanation for this question will be added shortly.We use "a" when we are referring to a single, non-specific item or thing. It is known as the indefinite article. In the context of your previous sentence, "a" is used before the word "pencil" to indicate that you borrowed one pencil from the pile, but the specific pencil is not being identified or mentioned.