Select the option that represents the letters that, when sequentially placed from left to right in the blanks below, will complete the letter series. _ D R G _ Z D R _ B Z D _ G B Z _ R G B

Correct — B. Twenty letters, five distinct symbols. Split the string into groups of five and it stops looking random.

Rule: the five-letter group Z D R G B repeats four times.

Group 1: _ D R G _ needs Z and B.
Group 2: Z D R _ B needs G.
Group 3: Z D _ G B needs R.
Group 4: Z _ R G B needs D.

Read the fills left to right: Z, B, G, R, D — ZBGRD, option (b).

Why the others are wrong

• (a)
  ZBGRB — The first four fills are right and the last one is B. That blank sits inside Z _ R G B, where D is the letter missing — a B there would put two Bs in one group.
• (c)
  BBGRD — It opens with B, making the series start B D R G B. Every complete group in the string begins with Z, so the first one must too.
• (d)
  ZBDRD — Its third fill is D, dropped into Z D R _ B where G is missing. That group would read Z D R D B, with D twice and no G at all.

Letter-gap series come in two shapes. One shifts every letter by a fixed step; the other, like this one, hides a repeating group.

Test for the second before you start counting alphabet positions. Count the letters, then look for a group length that divides them. Here there are twenty letters and five distinct symbols — Z, D, R, G, B — so groups of five are the first thing to try.

Once the length is right, each blank takes whichever member of the group is absent from its own group. No alphabet arithmetic is needed anywhere.

Key facts

• The series is the five-letter group ZDRGB written four times, twenty letters in all.
• The blanks fall at positions 1, 5, 9, 13 and 17.
• The fills, in order, are Z, B, G, R and D.

Common traps

• Reading the string as one long alphabet pattern and hunting for a step between neighbours.
• Assuming the last blank must be B because B closes every complete group you can see, when position 17 is a second-place slot.
• Losing count of the letters, so the group length comes out as four or six.

No directly related past PYQ was found.