The word "TETR" is spelled using four letters t-e-t-r. The pronunciation of this word follows a simple rule: the first letter "t" is pronounced as a voiceless alveolar plosive, represented in IPA phonetic transcription by /t/. The subsequent three letters are pronounced as a diphthong with an open-mid front unrounded vowel sound "/ɛ/" and a retroflex rhotic "ɻ", represented in IPA phonetic transcription by /ɛɻ/. Therefore, the correct pronunciation of the word "TETR" is /tɛɻ/.