I'm confused...

Greetings everyone. It has been a long time since I was active here.

I have a question regarding with song accompaniment. I was under the impression if I am using a C harp in second position, the song has to be in the key of “D”. Now, I am hearing that the music should be in the key of “G”. Which is it? ???

C harp in 2nd position is key of G
C harp in 3rd position is key of D
G harp in 2nd position is key of D

Thank you for the clarification.