Sure, let's analyze the underlined parts of each word to find the one with a different sound:

1. A. skytrain B. hyperloop C. cycling D. friendly
- The underlined part is "y" in all words.
- A. skytrain: /aɪ/
- B. hyperloop: /aɪ/
- C. cycling: /aɪ/
- D. friendly: /i/
- **Answer: D. friendly**

2. A. comfortable B. convenient C. economical D. autopilot
- The underlined part is "o" in all words.
- A. comfortable: /ʌ/
- B. convenient: /ə/
- C. economical: /ɪ/
- D. autopilot: /oʊ/
- **Answer: D. autopilot**

3. A. mine B. driver C. his D. find
- The underlined part is "i" in all words.
- A. mine: /aɪ/
- B. driver: /aɪ/
- C. his: /ɪ/
- D. find: /aɪ/
- **Answer: C. his**

4. A. traffic B. same C. crash D. jam
- The underlined part is "a" in all words.
- A. traffic: /æ/
- B. same: /eɪ/
- C. crash: /æ/
- D. jam: /æ/
- **Answer: B. same**

5. A. let B. jet C. vehicle D. success
- The underlined part is "e" in all words.
- A. let: /ɛ/
- B. jet: /ɛ/
- C. vehicle: /iː/
- D. success: /ə/
- **Answer: C. vehicle**

So, the words with different sounds in the underlined parts are:
1. D. friendly
2. D. autopilot
3. C. his
4. B. same
5. C. vehicle