In the following question, which one set of letters when sequentially placed at the gaps in the given letter series shall complete it?p _ r _ p _ q r _ p _ _

The given series has 12 character spaces in total (letters + blanks):p _ r _ p _ q r _ p _ _Let's test the pattern by dividing the series into 4 repeating blocks of 3 letters each (p q r):
$$\text{Block 1: }\text{p }\mathbf{q}\text{ r}$$
$$\text{Block 2: }\mathbf{p}\text{ p }\mathbf{q}$$
(doesn't repeat cleanly)Let's alternatively check option A (qpqrqr):Substituting q, p, q, r, q, r into the blanks sequentially:p q r p p q q r r p q rThis forms a symmetrical pattern of triplets: pqr, ppq, qqr, pqr which doesn't follow standard cycles.Let's test if the core repeating pattern is simply a repeating triplet of pqr:1st blank = q $\to$ p q r2nd blank = p $\to$ p p ...Let's re-verify the substitution pattern using option B (qpqrpq):p q r p p q q r r p p q $\to$ Pattern: pqr, ppq, qrr, ppq.Let's look at the continuous circular shift approach pqr:If we fill the gaps using option A (qpqrqr):p q r | p p q | q r r | p q r $\to$ This doesn't form a completely uniform repeating block.Let's review Option A again:The series can be split into 4 triplets: p_r, _p_, qr_, p__.If we insert Option A (q, p, q, r, q, r):p q rp p qq r rp q rThis can be understood as:1st block: pqr2nd block: ppq (doubling first term)3rd block: qrr (doubling last term)4th block: pqr (restarting cycle)Thus, the required letter set is qpqrqr.