Which of the following letter-clusters will replace the question mark (?) in the given series?BCDA, FGHE, JKLI, ?, RSTQ

Let's check the position-wise letter shift in each cluster across the series:1st letter: B $\to$ F $\to$ J $\to$ ? $\to$ R$\text{B}(2)+4=\text{F}(6)$, $\text{F}(6)+4=\text{J}(10)$, $\text{J}(10)+4=\mathbf{N}(14)$, $\text{N}(14)+4=\text{R}(18)$ 2nd letter: C $\to$ G $\to$ K $\to$ ? $\to$ S$\text{C}(3)+4=\text{G}(7)$, $\text{G}(7)+4=\text{K}(11)$, $\text{K}(11)+4=\mathbf{O}(15)$, $\text{O}(15)+4=\text{S}(19)$ 3rd letter: D $\to$ H $\to$ L $\to$ ? $\to$ T$\text{D}(4)+4=\text{H}(8)$, $\text{H}(8)+4=\text{L}(12)$, $\text{L}(12)+4=\mathbf{P}(16)$, $\text{P}(16)+4=\text{T}(20)$ 4th letter: A $\to$ E $\to$ I $\to$ ? $\to$ Q$\text{A}(1)+4=\text{E}(5)$, $\text{E}(5)+4=\text{I}(9)$, $\text{I}(9)+4=\mathbf{M}(13)$, $\text{M}(13)+4=\text{Q}(17)$ Combining the letters gives NOPM.