What was the perfect answer? Ban transgender people from pubic bathrooms? Pass a nationwide law that all public facilities must include a gender neutral option? He said leave it as it is. People use the bathroom they feel appropriate - which using logic means if it's not appropriate they will be reported just as they are now.
If you're a conservative, a good answer goes like this: "I'm not OK with it, but the states and locales should decide."
But... Trump isn't a conservative.