Step-by-step Solution:

Pattern Analysis

The pattern in this series is found by treating the letter positions in each four-letter group as four independent arithmetic sequences.

• 
  First Letter Position: 4 (D), 8 (H), ?, 16 (P), 20 (T)
  
  The pattern is +4. The missing position is 8 + 4 = 12, which corresponds to the letter L.
  


• 
  Second Letter Position: 3 (C), 7 (G), ?, 15 (O), 19 (S)
  
  The pattern is +4. The missing position is 7 + 4 = 11, which corresponds to the letter K.
  


• 
  Third Letter Position: 24 (X), 20 (T), ?, 12 (L), 8 (H)
  
  The pattern is -4. The missing position is 20 - 4 = 16, which corresponds to the letter P.
  


• 
  Fourth Letter Position: 23 (W), 19 (S), ?, 11 (K), 7 (G)
  
  The pattern is -4. The missing position is 19 - 4 = 15, which corresponds to the letter O.
  

Combining the letters for the missing third term gives us LKPO.

Final Answer: D. LKPO