To solve the problem of finding the missing letter cluster that completes the series, we need to analyze the sequence of given letter clusters: AZFX, EWHW, ?, MQLU, QNNT.

1. Firstly, let's break down each letter cluster into their individual letters:
   • AZFX
   • EWHW
   • ? (To be found)
   • MQLU
   • QNNT
2. We will now examine the pattern for each position in the clusters:
3. First Position:
   • A (1st letter of alphabet) → E (5th letter) → ? → M (13th letter) → Q (17th letter)
   • Observe a pattern: \(1 \rightarrow 5 \rightarrow 9 \rightarrow 13 \rightarrow 17\)
   • The letters increase by 4 positions ahead (A + 4 = E, E + 4 = I, I + 4 = M, M + 4 = Q)
   • Thus, the missing letter should be I.
4. Second Position:
   • Z (26th letter) → W (23rd letter) → ? → Q (17th letter) → N (14th letter)
   • Observe a pattern: \(26 \rightarrow 23 \rightarrow 20 \rightarrow 17 \rightarrow 14\)
   • The letters decrease by 3 positions (Z - 3 = W, W - 3 = T, T - 3 = Q, Q - 3 = N)
   • Thus, the missing letter should be T.
5. Third Position:
   • F (6th letter) → H (8th letter) → ? → L (12th letter) → N (14th letter)
   • Observe a pattern: \(6 \rightarrow 8 \rightarrow 10 \rightarrow 12 \rightarrow 14\)
   • The letters increase by 2 positions (F + 2 = H, H + 2 = J, J + 2 = L, L + 2 = N)
   • Thus, the missing letter should be J.
6. Fourth Position:
   • X (24th letter) → W (23rd letter) → ? → U (21st letter) → T (20th letter)
   • Observe a pattern: \(24 \rightarrow 23 \rightarrow 22 \rightarrow 21 \rightarrow 20\)
   • The letters decrease by 1 position (X - 1 = W, W - 1 = V, V - 1 = U, U - 1 = T)
   • Thus, the missing letter should be V.
7. Putting it all together, the missing letter cluster is "ITJV".

Therefore, the letter cluster that completes the series is ITJV. This matches the correct answer provided.