To find the missing term in the given sequence, let's analyze the pattern:

1 BR, 2 EO, 6 HL, 15 KI, ?

Observations:
- The first letter of each term follows a pattern: B, E, H, K.
- The second letter of each term follows a pattern: R, O, L, I.
- The number of the term is related to the second letter: 1, 2, 6, 15.

Pattern Analysis:
- The first letter of each term is increasing by 3: B + 3 = E, E + 3 = H, H + 3 = K.
- The second letter of each term is decreasing by 3: R - 3 = O, O - 3 = L, L - 3 = I.
- The number of the term is increasing by the sum of the positions of the second letter in the alphabet: R (18th letter) + O (15th letter) = 33, O (15th letter) + L (12th letter) = 27, L (12th letter) + I (9th letter) = 21.

Using these patterns, we can determine the missing term:

The fourth term is 15 KI, so the second letter is "I" (9th letter). The number of the fourth term is 15, and the position of "I" in the alphabet is 9. Hence, the sum is 15 + 9 = 24.

Now, let's find the fifth term:
- The first letter of the fifth term is K + 3 = N.
- The second letter of the fifth term is I - 3 = F.
- The number of the fifth term is 15 + 24 = 39.

Therefore, the missing term is 39 NF, which corresponds to option (b) 31 NF.