Answer: D. Kira used published data that is quantitative.
The published data refers to the government census info being published online. She did not do an observational study because she didn't go to each household herself (rather let the census workers do that task).
The data is quantitative because she is looking at the number of people living in homes in her city. Quantitative data is simply dealing with quantities or numbers. Contrast this with qualitative data that talks about something like color choice.