Which letter cluster will replace the question mark (?) to complete the given series?
$$\text{WTNM, SYHT, ? , KIVH, GNPO}$$

Let us track the mathematical positional differences ($A=1,B=2,\dots ,Z=26$) for each letter slot in the clusters across the series:
1. First letter slot pattern:

$$\text{W}(23)\stackrel{-4}{\to }\text{S}(19)\stackrel{-4}{\to }\mathbf{O}(15)\stackrel{-4}{\to }\text{K}(11)\stackrel{-4}{\to }\text{G}(7)$$

2. Second letter slot pattern:

$$\text{T}(20)\stackrel{+5}{\to }\text{Y}(25)\stackrel{+5}{\to }\mathbf{D}(4)\stackrel{+5}{\to }\text{I}(9)\stackrel{+5}{\to }\text{N}(14)(\text{Note: }25+5=30\equiv 4=\text{D})$$

3. Third letter slot pattern:

$$\text{N}(14)\stackrel{-6}{\to }\text{H}(8)\stackrel{-6}{\to }\mathbf{B}(2)\stackrel{-6}{\to }\text{V}(22)\stackrel{-6}{\to }\text{P}(16)(\text{Note: }8-6=2=\text{B},\text{ and }2-6=-4\equiv 22=\text{V})$$

4. Fourth letter slot pattern:

$$\text{M}(13)\stackrel{+7}{\to }\text{T}(20)\stackrel{+7}{\to }\mathbf{A}(1)\stackrel{+7}{\to }\text{H}(8)\stackrel{+7}{\to }\text{O}(15)(\text{Note: }20+7=27\equiv 1=\text{A})$$

Combining the derived letters from each calculation gives ODBA.