The spelling of the word "Luthor" can be explained using the International Phonetic Alphabet (IPA). The first syllable is pronounced /lu:/ which is a long "U" sound. The "th" in the second syllable is pronounced as a voiceless dental fricative /θ/ and the final syllable is pronounced as /ər/, which is the schwa sound followed by the "R" sound. Therefore, the correct spelling of the word is "Luthor" with a long "U" sound in the first syllable and a voiceless "TH" in the second syllable.