I know that you can use ka, no, and noka as question markers. But what is "na" in "hontou na no ka" . is it another form of question marker altogether or what does it mean the "na" in that sentence?
な is not part of the question marker because the same な can appear in non-question sentences. This な is an attributive form of the copula だ. (Chart: Conjugations of the Japanese Copula)
の here is a special "noun" known as explanatory の. Thus だ before it needs to be turned into な to modify a noun. As you can see in the link, all predicative expressions in their attributive form can precede の.
- 赤いのか。 Is it that it's red? (i-adjective)
- 簡単なのか。 Is it that it's easy? (na-adjective)
- 動くのか。 It it that it moves? (verb)
- 本なのか。 Is it that it's a book? (noun + copula)
本当 is a noun which is also categorized as a no-adjective, but we always say 本当なのか, not 本当ののか.