な+の (attributive copula)

When a noun or な-adjective appears before the explanatory/nominalizing の, the copula surfaces as the attributive な, not だ — 本当なの? ('is it really true?'), 学生なの ('it's that he's a student'), 問題なの ('what's the problem?'). A frequent point of learner confusion, because だ is the default copula but だ+の is ungrammatical; the attributive な is required here.