Emphatic stress is the placement of greater stress on a particular word in an utterance. This is done for clarification, emphasis or correction. Stress is placed on the word "my" to correct an error made by the person who asked the question. The person who asked the question mistakenly assumes that it was "his" pen that the teacher wants, but the responder clarifies by saying that it is "my" pen the teacher wants, not "his". 

Q: Does the teacher want his pen?

A: No the teacher wants MY pen.