Which of the following is an example of a word containing the 'ɝ' sound?

The 'ɝ' sound is a mid-central vowel that is characterized as a stressed rhotacized vowel, often found in English words that include the combination of the schwa and an 'r'. It is commonly heard in words with a stressed syllable that includes the letter 'r'. In this case, the word "bird" contains the 'ɝ' sound, pronounced as /bɜrd/ in phonetic transcription, which features the characteristic rounded sound associated with the 'ɝ' symbol.

Words like "bud," "about," and "summer" do not contain the 'ɝ' sound. "Bud" features a short vowel sound /ʌ/, "about" has an unstressed schwa /ə/ in the first syllable, and "summer" contains the vowel sound /ʌ/ along with a final syllabic /ɚ/. Therefore, "bird" is the clear choice as it exemplifies the presence of the 'ɝ' sound.