Select the option that best completes the letter analogy: ABC : BDF :: JKL : ?

The three letters do not move by the same amount: the first advances 1, the second 2 and the third 3. A becomes B, B becomes D and C becomes F. Applying the growing steps to J, K, L gives K, M, O. 'KLM' is the trap for a solver who assumes a single uniform step of 1, 'MNO' assumes a uniform step of 3, and 'KMP' gets the first two right but pushes the last letter 4 places instead of 3.