Here are the correct options for each statement:

1. **so**
- The city's public transportation system is **so** affordable that many residents prefer using it.

2. **quickly**
- The population of the city is rising **so quickly** that there aren't enough jobs.

3. **so**
- The environment is getting **so** polluted that we will have serious health problems.

4. **serious**
- The traffic jams were **so serious** that the city decided to widen streets.

5. **such**
- Homelessness is **such** a serious problem that the city has to take action to solve it.