Answer: b) Return the pen to your colleague and apologize for the mistake.
Step-by-step explanation:
Returning the pen to your colleague and apologizing for the mistake is the most ethical course of action because it shows honesty, respect, and accountability. It is important to acknowledge and rectify the situation when you realize that you have taken something that belongs to someone else without their permission. By taking responsibility for your actions and making amends, you demonstrate integrity and uphold ethical values.