A series is given with one term missing. Choose the correct alternative from the given ones that will complete the series.
?, YZ, DE, JK

Let's analyze the difference between the first letters of consecutive pairs in the series:
Let the missing term be represented as Pair 1.

Pair 2: YZ (First letter = Y (25))

Pair 3: DE (First letter = D (4, or 30 in cyclic order))

Pair 4: JK (First letter = J (10, or 36 in cyclic order))

Let's find the difference pattern between the first letters of the known elements:

From Y to D: 30−25=+5

From D to J: 36−30=+6

Following this increasing gap pattern (+4,+5,+6), the difference between the first letter of Pair 1 and Pair 2 (Y) must be +4:

Position of first letter+4=25⟹Position=21→U
Since all clusters consist of two consecutive alphabets (YZ, DE, JK), the cluster starting with U is UV.