In the given pattern, following points can be observed.
1. The numbers are decreasing. The next number in the pattern will be 1 less than the previous one.
2. The Alphabets with the numbers are in increasing order, i.e, A, B, C, ....
Therefore based on the above observation
1. Number will be 23, which is 1 less than 24
2. D will be the alphabet, which comes after C in the alphabet.
Thus, 23D will come in the blank. So, the complete pattern will be:
26A, 25B, 24C, 23D, 22E, 21F