Consider the sequence A _ BCD _ BBCDABC _ DABC _ D that follows a certain pattern. Which one of the following completes the sequence?

The correct option is A, A, C, D.

To find the pattern, let us write down the given incomplete sequence:
A _ B C D _ B B C D A B C _ D A B C _ D

The total length of the sequence is 20 letters. We can divide this sequence into 4 equal groups of 5 letters each:
1. First Group: A _ B C D
2. Second Group: _ B B C D
3. Third Group: A B C _ D
4. Fourth Group: A B C _ D

Now, let us substitute the letters from the correct option A, A, C, D into the respective blanks:
- First blank filled with A
- Second blank filled with A
- Third blank filled with C
- Fourth blank filled with D

Substituting these values gives us the following completed groups:
1. First Group: A A B C D (The letter A is repeated twice)
2. Second Group: A B B C D (The letter B is repeated twice)
3. Third Group: A B C C D (The letter C is repeated twice)
4. Fourth Group: A B C D D (The letter D is repeated twice)

This reveals a clear and systematic pattern: in each consecutive group of 5 letters (A, B, C, D), a different letter is doubled in alphabetical order (first A, then B, then C, and finally D). Therefore, the sequence is completed by the letters A, A, C, D.