The word "CBFK" isn't found in any dictionary, and therefore doesn't have an established spelling. However, phonetically, it could be transcribed as /si-bi-ɛf-keɪ/ or /si-bi-ɛf-kaɪ/ depending on if the "K" is pronounced with a soft "k" sound (as in "kite") or a hard "k" sound (as in "kangaroo"). Without further context, it's difficult to determine the intended spelling or pronunciation of this made-up word.