To find the unit rate for all 24 problems, we first need to find the total time Deandra took to complete all the problems.
Deandra completed the first 12 problems in 1 hour and the last 12 problems in 3 hours.
So the total time she took is:
1 hour + 3 hours = 4 hours
Now, we can calculate the unit rate by dividing the total number of problems by the total time:
\[ \text{Unit Rate} = \frac{\text{Total Problems}}{\text{Total Time}} = \frac{24 \text{ problems}}{4 \text{ hours}} = 6 \text{ problems per hour} \]
Therefore, the correct answer is 6 problems per hour.