Explain ㄴ/는지 v.s. ㄹ/을지 v.s. ㄹ/을까.
I am wondering if there are any differences between A and C.
And out of A an B, which one sounds more natural?

A 어떻게 해야 하는지 걱정이다.
B 어떻게 해야 할지 걱정이다.
C 어떻게 해야 할까 걱정이다.

A detailed explanation will be appreciated.
Thanks a lot!

Question

Explain ㄴ/는지 v.s. ㄹ/을지  v.s. ㄹ/을까.
I am wondering if there are any differences between A and C.
And out of A an B, which one sounds more natural?

A 어떻게 해야 하는지 걱정이다.
B 어떻게 해야 할지 걱정이다.
C 어떻게 해야 할까 걱정이다.

A detailed explanation will be appreciated.
Thanks a lot!

Answer

하는지: you are thinking/worrying about what would the right/best thing to do.
할지: you are thinking/worrying about how you could solve certain problems.
할까: similar to above, however, you have a small selection of concrete options in this case similar to answering a multiple choice question.