The logic is:
P_rsqrps_pqs_qr_qrps_p_s
Dividing the series in group of 4 letters :
(p_rs) (qrps) (_pqs) (qr) (qrps) (_p_s)
We can see that the 2nd and 5th terms are same in the series.
So, accordingly, 1st and 4th term, 2nd and 5th and 3rd and 6th terms must be same.
So, on comparing 1st and 4th terms, (p_rs) and (qr), we can easily recognise that the blank in 1st term will have ‘q’ and the two blanks in 4th term will be filled by ‘p’ and ‘s’ respectively.
Similarly,
3rd and 6th term, on comparison, (_pqs) and (_p_s), we find that the 2nd blank in 6th term will be filled by ‘q’ and the 1st blank in both the terms will be filled by ‘r’, as each term is made up od using all the four terms p, q, r and s.
So, following the pattern, we find that ‘q, r, p, s, r, q’, when filled in the blanks, will complete the given series.
Pqrsqrpsrpqspqrsqrpsrpqs
Hence, ‘q, r, p, s, r, q’ is the correct answer.