Here the rule which is applicable on the series: 9 18 27 36 is Rule E: Divide by 2 and multiply by 3. as:
Start with the number 6.
6÷2=3,3×3=9
Now, take the number 12.
12÷2=6,6×3=18
Take the number 18.
18÷2=9,9×3=27
Finally move on to the number 24.
24÷2=12,12×3=36
Hence, Rule (E) is the correct answer.