Step 1: Idea:
Look at the numbers in odd places and even places separately.
Step 2: Odd places:
The numbers in the 1st, 3rd, 5th and 7th places are 5, 9, 11, 13. These are odd numbers in increasing order.
Step 3: Even places:
The numbers in the 2nd, 4th, 6th and 8th places are 15, ?, 33, 39. Divide the known ones by the number just before them: 15 / 5 = 3, 33 / 11 = 3, 39 / 13 = 3.
Step 4: Fill the blank:
The number before the blank is 9, so the blank is 3 x 9 = 27. This matches option 3.
Final Answer:
The answer is 27.
\[ \boxed{27} \]