Concept:The underlined letters th in "then" make the voiced dental sound /ð/.Explanation:"Then" is pronounced /ðen/.The sound is a voiced consonant, produced with vibration of the vocal cords.Check each option:"thaw" begins with the voiceless /θ/ sound."smooth" contains the voiced /ð/ sound at the end."den" starts with /d/, not /ð/."wealthy" has the voiceless /θ/ sound.Only "smooth" shares the same voiced consonant sound as "then".Answer:B. smooth