B2CD, _____, BCD4, _____, BC6D

In the given series, the alphabets are same and only the number is changing. By observation, we see that the number is increased by 1 in each subsequent term. Also, the position of number changes by 1 in each subsequent term and this change is cyclic. In the first term, the number 2 is at the second position. So, in the third term, the number should be at the third position and it must be increased by 1. Thus, the third term must be BC3D.
In the fourth term, the number is 4 and is at the last. So, in the fifth term, it should be at the second position (as the change in position is cyclic) and must be increased by 1. Thus, the fourth term will be B5CD.
Hence, option A is the correct answer.