Good-bye, good-by, goodbye, goodby?

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.
a. good-bye
b. good-by
c. goodbye
d. goodby

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.

@EditorMark argues for saying goodbye to the hyphen and reports on various guides:

“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.’”

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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.”