What comes next in the series:
$$\text{Z, W, R, K, ?}$$

Let's note the numerical positions of the letters in alphabetical order ($\text{A}=1,\text{B}=2,\dots ,\text{Z}=26$):
* $\text{Z}=26$
* $\text{W}=23$ (Difference: $26-3=23$)
* $\text{R}=18$ (Difference: $23-5=18$)
* $\text{K}=11$ (Difference: $18-7=11$)

The pattern of differences subtracted is consecutive odd numbers starting from $3$: $-3,-5,-7$.
Following this logic, the next difference must be $-9$:

$$\text{Next Position}=11-9=2$$

The letter at position $2$ is B.