A common way to title a response is to tack on "[Plaintiff/Defendant's] Response to [Defendant/Plaintiff's] Motion to [Whatever their motion was called]".
If you want to know if you're entitled to respond to a specific pleading, and can't get an answer from the court clerk, check the rules of court. I am hesitant to describe your rights in relation to a document I can't review. Note that the usual consequence for filing an answer to a pleading you're not entitled to answer is that the court doesn't review it; and sometimes the court will review it anyway.

