Answer: D
Correct answer: J
Every letter is two places further on than the one before it: B to D, D to F, F to H. Two places past H is J. The series is simply the even-numbered letters of the alphabet - positions 2, 4, 6, 8, 10. The trap is I, which is the very next letter but only one step on; the gap here has been two throughout and there is nothing in the series to suggest it changes. Series built on a single constant gap are the foundation of this topic, and every harder pattern here is a variation on one: alternating gaps, growing gaps, or two such series interleaved.