Let's analyze the pattern in the given sequence:
- The first letter alternates between lowercase and uppercase:
- a, b, c, d, and then the next would be e.
- The second letter follows the pattern: B, B, D, D.
The letter repeats for two terms, then increases by 2 (B → D). The next letter in the pattern is F.
Thus, the missing term is eF.
Therefore, the correct answer is \( \boxed{eF} \).