Concept:The letters "th" can be pronounced as a voiced sound or a voiceless sound. We must choose the option with the same "th" sound as in "then".
Explanation:In "then", the underlined "th" is voiced, pronounced as /ð/.
This is the same consonant sound heard in words like "this", "that", and "mother".
Option A, "month", uses the voiceless /θ/ sound, like in "bath".
Option B, "three", also uses the voiceless /θ/ sound.
Option C, "thunder", has the voiceless /θ/ sound as well.
Option D, "thus", is pronounced /ðʌs/, with the voiced /ð/ sound.
Therefore, "thus" matches the voiced "th" sound found in "then".
Answer:D. thus