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Answer: (D) ZBGRD
Explanation
The letter series is based on the continuous repetition of a five-letter unit: Z, D, R, G, B. The entire sequence consists of 20 characters, which can be broken down into four identical blocks of five letters each.
Block 1: Z D R G B Block 2: Z D R G B Block 3: Z D R G B Block 4: Z D R G B
By comparing this repeating pattern to the provided string _DRG_ZDR_BZD_GBZ_RGB, the missing letters at the blank positions 1, 5, 9, 13, and 17 are determined. Position 1 requires Z, position 5 requires B, position 9 requires G, position 13 requires R, and position 17 requires D. Placing these letters sequentially results in the set ZBGRD, which completes the uniform periodic sequence ZDRGBZDRGBZDRGBZDRGB.