To find the word which has a different sound in the part underlined, let's analyze each set of words:

1. **ion 1:**
- A. dear
- B. fear
- C. hear
- D. heart

The underlined part in "heart" has a different sound compared to the others. The others have the "ear" sound, while "heart" has the "ar" sound.

2. **ion 2:**
- A. stupid
- B. studio
- C. study
- D. student

The underlined part in "study" has a different sound. The others have the "u" sound as in "you," while "study" has the "u" sound as in "uh."

3. **ion 3:**
- A. brother
- B. thick
- C. they
- D. that

The underlined part in "brother" has a different sound. The others have the "th" sound, while "brother" has the "th" sound as in "the."

4. **ion 4:**
- A. machine
- B. parachute
- C. champagne
- D. cheer

The underlined part in "cheer" has a different sound. The others have the "sh" sound, while "cheer" has the "ch" sound.

5. **ion 5:**
- A. doctor
- B. hospital
- C. pollution
- D. tomorrow

The underlined part in "pollution" has a different sound. The others have the "o" sound, while "pollution" has the "u" sound.

6. **ion 6:**
- A. great
- B. treasure
- C. measure
- D. pleasure

The underlined part in "great" has a different sound. The others have the "zh" sound, while "great" has the "ai" sound.

7. **ion 7:**
- A. ancient
- B. cave
- C. cavern
- D. aging

The underlined part in "cave" has a different sound. The others have the "a" sound as in "cat," while "cave" has the "a" sound as in "day."

8. **ion 8:**
- A. spectacular
- B. structure
- C. sculpture
- D. picturesque

The underlined part in "picturesque" has a different sound. The others have the "u" sound, while "picturesque" has the "e" sound.

9. **ion 9:**
- A. shrine
- B. surprising
- C. pilgrim
- D. island

The underlined part in "island" has a different sound. The others have the "i" sound, while "island" has the silent "s."

10. **ion 10:**
- A. structure
- B. culture
- C. sculpture
- D. future

The underlined part in "future" has a different sound. The others have the "u" sound, while "future" has the "yu" sound.

11. **ion 11:**
- A. cavern
- B. citadel
- C. complex
- D. contestant

The underlined part in "complex" has a different sound. The others have the "e" sound, while "complex" has the "o" sound.