Here are the words where the underlined parts are pronounced differently from the rest:

1. **C. sea** - In this option, the "ea" is pronounced as /iː/, while the "ea" in "idea" and "beach" are pronounced as /i:/, and "breathe" has a different vowel sound altogether.

2. **D. marathon** - The "th" in "marathon" is pronounced differently (as /θ/) compared to the other words where "th" is pronounced as /ð/.

3. **A. forest** - The "e" in "forest" is pronounced as /ɔː/, while the "e" in the other words is pronounced as /i:/.

4. **C. heavy** - The "ea" in "heavy" is pronounced differently (as /ɛ/) compared to the other words where "ea" is pronounced as /iː/.

5. **A. changes** - The "e" at the end of "changes" is pronounced as /ɪz/, while the other words have "es" pronounced as /ɪz/ as well, but "changes" is notably different because the base form changes the pronunciation.

6. **B. hand** - The "a" in "hand" is pronounced as /æ/, whereas the others have the /ɑː/ or /eɪ/ sounds.

7. **No distinct difference exists.** All the underlined parts "th" being pronounced consistently in "thank," "think," "through," and "then."

8. **No distinct difference exists.** All the underlined parts "ow" are pronounced as /aʊ/ or /oʊ/.

9. **C. sound** - "ou" has a consistent pronunciation with other words, making it difficult to determine a different pronunciation.

10. **C. cat** - The "a" in "cat" is pronounced as /æ/, which is distinct from the other words that have /eɪ/ or /ɑː/

I hope this helps! If you would like any further clarification, feel free to ask.