**Solution:** The correct answer is 39.

Explanation: The given series is 8, 15, 26, ?, 56, 75. Let’s analyze the pattern:

$$

\begin{array}{cccccc}

8 & 15 & 26 & 39 & 56 & 75 \\

\downarrow & \downarrow & \downarrow & \downarrow & \downarrow \\

+7 & +11 & +13 & +17 & +19 \\

\downarrow & \downarrow & \downarrow & \downarrow \\

+4 & +2 & +4 & +2 \\

\end{array}

$$

The pattern shows that each number is derived by adding a progressively changing difference. The differences between the consecutive numbers are +7, +11, +13, +17, +19, and the changes between these differences follow the pattern +4, +2, +4, +2. Therefore, the missing number in the sequence is **39**.