Select the combination of letters that when sequentially placed in the blanks will create a repetitive pattern.

pq_ppq_ppq_pp_pp

Given: pq_ppq_ppq_pp_pp

Let's check all the options:

(a) pppq (Correct)

Sequence: p q p p / p q p p /  p q p p / p q p p

(b) pqpq  (Incorrect)

Sequence: p q p p / p q q p /  p q p p / p q p p​

(c) qppp (Incorrect)

Sequence: p q q p / p q p p /  p q p p / p p p p​

(d) pqqp (Incorrect)

Sequence: p q p p / p q q p /  p q q p / p p p p​

Thus, the correct option is (a) pppq