Which letter cluster will replace the question mark (?) to complete the given series?
$$\text{HEWC, KIRI, ?, QQHU, TUCA}$$

Let's analyze the pattern for each position of the letter clusters by converting them to their positional values in the English alphabet:
* First letter: $\text{H}(8)\stackrel{+3}{\to }\text{K}(11)\stackrel{+3}{\to }\mathbf{N}(14)\stackrel{+3}{\to }\text{Q}(17)\stackrel{+3}{\to }\text{T}(20)$
* Second letter: $\text{E}(5)\stackrel{+4}{\to }\text{I}(9)\stackrel{+4}{\to }\mathbf{M}(13)\stackrel{+4}{\to }\text{Q}(17)\stackrel{+4}{\to }\text{U}(21)$
* Third letter: $\text{W}(23)\stackrel{-5}{\to }\text{R}(18)\stackrel{-5}{\to }\mathbf{M}(13)\stackrel{-5}{\to }\text{H}(8)\stackrel{-5}{\to }\text{C}(3)$
* Fourth letter: $\text{C}(3)\stackrel{+6}{\to }\text{I}(9)\stackrel{+6}{\to }\mathbf{O}(15)\stackrel{+6}{\to }\text{U}(21)\stackrel{+6}{\to }\text{A}(27\equiv 1)$

Combining these letters gives the missing cluster: NMMO.