How to respond to negative questions (adjectives)?

My friend got me this question:

When somebody asks, "Is it not available?"

Should I say:
(1) "Yes, it is not available." OR
(2) "No, it is not available."?

I know it would be better to use the word unavailable. I checked out another question titled "How to answer a negative question without ambiguity?" But it isn't about adjectives so I'm starting a new question. Thank you!