Here are the words whose underlined part differs from the other three in pronunciation:

1. **A. illustrate** - The underlined part "il" is pronounced as /ɪl/, while the "i" in the other options is pronounced as /aɪ/.

2. **B. connect** - The underlined part "ne" is pronounced as /nɛ/, differing from "prestige" and "crisis" where "e" is pronounced as /aɪ/.

3. **B. crisis** - The underlined part "i" is pronounced as /aɪ/, while in "housing" and "necessitate," the "i" is different.

4. **A. navigate** - The underlined part "a" is pronounced as /æ/, which differs from the "i" in "inadequate" and "convenience."

5. **D. regional** - The underlined part "re" is pronounced differently than the others where the "o" is pronounced differently.

So, to sum up:
1. A
2. B
3. B
4. A
5. D