Which set of letters, when placed in order in the blanks of the given…

The correct answer is Option (3) → dbcd

Such a series is usually one block of letters repeated. The series has 20 places, and the complete part “c d b c” (places 13 to 16) suggests a block of four letters.

• Split the series into blocks of four: c _ b c | c d _ c | c d b _ | c d b c | c _ b c

• Every block must read c d b c, so the blanks are d (first block), b (second block), c (third block) and d (fifth block)

• The completed series is cdbc cdbc cdbc cdbc cdbc, with the block repeated five times

So the letters for the blanks, in order, are dbcd.