Complete the series:
B E H, D G J, F I L, ?

The correct option is H K N.

To solve this letter series, let us analyze the pattern followed by the letters in each group step-by-step.

First, let us write down the numerical positions of each letter in the English alphabet:

$\text{B}=2,\text{E}=5,\text{H}=8$

$\text{D}=4,\text{G}=7,\text{J}=10$

$\text{F}=6,\text{I}=9,\text{L}=12$

Now, let us examine the pattern across the groups position by position:

1. First letter of each term:
B (2) ⇒ D (4) ⇒ F (6)
Notice that 2 is added to each step (2 + 2 = 4, 4 + 2 = 6).
The next first letter will be position 6 + 2 = 8, which corresponds to H.

2. Second letter of each term:
E (5) ⇒ G (7) ⇒ I (9)
Notice that 2 is added to each step (5 + 2 = 7, 7 + 2 = 9).
The next second letter will be position 9 + 2 = 11, which corresponds to K.

3. Third letter of each term:
H (8) ⇒ J (10) ⇒ L (12)
Notice that 2 is added to each step (8 + 2 = 10, 10 + 2 = 12).
The next third letter will be position 12 + 2 = 14, which corresponds to N.

Combining all three letters together gives H K N.