Answer is B, D, F, I, L, P, "T".
The letters count between B & D and D & F is "1".
Similarly between F & I and I & L is "2".
From this, we get same letter count between 2 combinations of letters in the series like {B&D , D&F} and {F&I , I&L}.
So for the next two series letter count { L&P and P&? } must be "3".
So 3 letters next to P is T.