Which is correct?
According to some lexicographers, we may soon be saying ______ to the word hello because so much communication these days is through social networking sites, such as Twitter and Facebook, where greetings tend to be less formal.
Answer: c, for AP style.
However, if you aren’t constrained by a style guide, you can probably get away with any variation you choose.
“The American Heritage and Webster’s New World dictionaries list goodbye as the first spelling. Bryan Garner in “Garner’s Modern American Usage,” compares the hyphenated form to the archaic “to-day.”
Merriam-Webster, though, includes only “good-bye” and “good-by.” Many style guides, including the Chicago Manual of Style, prefer a Merriam-Webster dictionary, so “good-bye” is with us for now.
The word in any form is only a few hundred years old, stemming from the earlier “good morning” and “good day,” etc., and a shortening of the phrase ‘God be with you.’”
Ed Hurley reported that both hello and goodbye might be on the way out back in 2003:
Pretty soon “hello” and “goodbye” may become as passe as “ye” and “thou.”