The spelling of the word "most balloon" can be explained using IPA phonetic transcription. The first syllable "most" is pronounced as /məʊst/. The "o" in "most" is pronounced with a long vowel sound as in the word "coat" /kəʊt/. The second syllable "balloon" is pronounced as /bəˈluːn/. The "oo" in "balloon" is pronounced as a long vowel sound as in the word "moon" /muːn/. Therefore, the correct spelling of this word is "most balloon" and it is pronounced as /məʊst bəˈluːn/.