Concept:The phonetic symbol /ð/ represents the voiced “th” sound, like the beginning of “their.”Explanation:Option A, “thought,” is pronounced /θɔːt/, so it uses the voiceless /θ/ sound.Option B, “den,” begins with the /d/ sound, not /ð/.Option C, “their,” begins with /ð/, which matches the given symbol.Option D, “thank,” uses the voiceless /θ/ sound at the start.Therefore, the word that contains the sound /ð/ is “their.”Answer:C. their