Hey there!
The answer to your first problem will be a line graph. These graphs represent data in relation to time on individual axes, which will be perfect to represent Karen's temperature data. A circle graph, histogram, and venn diagram will all deal with her data in relation to how often each average occurs, which isn't what she wants to do.
The answer to your second problem will be a histogram. Again, this graph deals with data in relation to how often each point occurs, but will be the best for the interval that Brittany wants to use.
Hope this helped you out! :-)