Which letter cluster can replace the question mark (?) to complete the given series?
$$\text{HDZH, IFWL, ?, KJQT, LLNX}$$

Let's analyze the pattern for each position of the letter clusters:First Letter:$\text{H}\stackrel{+1}{\to }\text{I}\stackrel{+1}{\to }\text{J}\stackrel{+1}{\to }\text{K}\stackrel{+1}{\to }\text{L}$ Second Letter:$\text{D}\stackrel{+2}{\to }\text{F}\stackrel{+2}{\to }\text{H}\stackrel{+2}{\to }\text{J}\stackrel{+2}{\to }\text{L}$ Third Letter:$\text{Z}\stackrel{-3}{\to }\text{W}\stackrel{-3}{\to }\text{T}\stackrel{-3}{\to }\text{Q}\stackrel{-3}{\to }\text{N}$ Fourth Letter:$\text{H}\stackrel{+4}{\to }\text{L}\stackrel{+4}{\to }\text{P}\stackrel{+4}{\to }\text{T}\stackrel{+4}{\to }\text{X}$ Combining all the characters, the missing cluster is JHTP.