Select the combination of letters that when sequentially placed in the blanks of the given series will complete the series.
P_L_Q_LM_P_M_Q_M

Let's find the total count of letters including blanks. There are 16 positions.
Let's divide the series into 4 repeating clusters of 4 letters each:

P _ L _ ∣ Q _ L M ∣ _ P _ M ∣ _ Q _ M
Let's look for standard patterns across groups or a shifting sequence.
Let's substitute Option C: L, Q, L, M, P, Q, P

Position 2: L → PLL_

Position 4: Q → PLLQ

Position 6: L → QLLM

Position 9: M → _P_M → MP_M

Position 11: P → MPPM

Position 13: Q → _Q_M → QQ_M

Position 15: P → QQPM

Let's test Option D: P, M, Q, P, L, Q, L
Let's look at the patterns formed by checking if there's a simpler cycle:
What if the string is broken into groups of 4:
If we look at option D:

P P L M | Q Q L M | P P L M | Q Q L M
Let's look at this pattern:

Group 1: P P L M

Group 2: Q Q L M

Group 3: P P L M

Group 4: Q Q L M

The sequence of blanks filled is: P, M, Q, P, L, Q, L.
This forms a clean, perfectly alternating structural cycle of PPLM and QQLM.