If somebody asks a question in the singular, how do we give a plural answer? Here's an example, with three different possible ways to answer. (The part of the answer that has the plural sense is marked in bold):
Q: What's the most important thing to you?
A1: The most important thing to me is my friends.
A2: The most important thing to me are my friends.
A3: The most important things to me are my friends.
I am a native speaker of British English. I prefer A1. An American told me she preferred A2. I suspect that a formal prescriptivist would recommend A3.
Is there any rhyme or reason to this? What should I recommend to a student learning English?