Select the letter-cluster from among the given options that can replace the question mark (?) in the following series.
$$\text{NIR, JGN, FEJ, ?}$$

Let's analyze the pattern for each position in the letter-clusters by converting them to their positional values in the English alphabet:First letter of each cluster:$N(14)\to J(10)\to F(6)\to ?$ The pattern is: $14-4=10$, $10-4=6$, $6-4=2$.The letter with positional value $2$ is B.Second letter of each cluster:$I(9)\to G(7)\to E(5)\to ?$ The pattern is: $9-2=7$, $7-2=5$, $5-2=3$.The letter with positional value $3$ is C.Third letter of each cluster:$R(18)\to N(14)\to J(10)\to ?$ The pattern is: $18-4=14$, $14-4=10$, $10-4=6$.The letter with positional value $6$ is F.Combining these, the missing letter-cluster is BCF.