Select the combination of letters that when sequentially placed in the blanks of the given series will complete the series. q p _ r _ r q _ p _ r p _ q _ p r _ p _ q

Correct — C. Rule: the seven-letter unit q p r r p r q, repeated three times.

The series has 21 places and 8 blanks. Cut it after every seventh letter:

q p _ r _ r q
_ p _ r p _ q
_ p r _ p _ q

Stack the three rows and read down each column. The printed letters never clash, and together they give q p r r p r q, so every blank is fixed by its position in that unit.

Reading the blanks in order: r, p, q, r, r, q, r, r — option (c).

Why the others are wrong

• (a)
  r p q r r p r q — This matches for five blanks, then puts p at the sixth, where the third repetition has to restart with q. Its final blank is q where the pattern needs r.
• (b)
  r p q r r q r p — Right until the last blank, where it puts p. That slot is the sixth letter of the third repetition and must be r — p already stands one place before it.
• (d)
  r p q q r q r p — It fails at the fourth blank, putting q where the unit's third letter r belongs, and then repeats the same wrong ending as option (b).

A blank-filling letter series like this one is one short unit written out over and over. Test for that first: fix the unit's length, then fill by position instead of guessing letter by letter.

There are two routes to the length. Count the places and factorise — 21 places points to a unit of 3 or 7. Or look for a landmark: q stands at places 7, 14 and 21, which fixes the length at 7.

Once the length is known, stack the segments and read down the columns. A letter printed in any one segment fills that column in all three.

Key facts

• The completed series is q p r r p r q written three times, 21 letters in all.
• In the completed series the letter q stands at places 1, 7, 8, 14, 15 and 21.
• Stacking equal-length segments and reading down the columns fills every blank without guessing.

Common traps

• Filling blanks left to right by feel instead of fixing the unit length first
• Testing an option on its first three blanks, which every option survives
• Miscounting the places, which shifts every blank by one

No directly related past PYQ was found.