The correct spelling is "was most prevalent", with the /wʌz/ sound for "was" and the /moʊst/ sound for "most". The common mistake of spelling "wast prevalent" may be due to the archaic use of "wast" as the second person singular past tense of "be". However, the correct usage in modern English is "were" for plural and "was" for singular. It's crucial to pay attention to correct spelling and pronunciation to avoid confusion and ambiguity in communication.