How should I answer Protestants who ask whether I am a Christian?
The short answer
One good answer is, "Yes, I'm a Catholic." It affirms that you are a Christian and tells them that you are Catholic and regard Catholics as Christians.
A simple reply is: "Yes, I'm a Catholic."
Your non-Catholic friends may not mean to insult you; the question may be the start of "witnessing" to you. Saying "yes" affirms that you are a Christian. Adding that you are Catholic tells them your affiliation and that you consider Catholics to be Christians.