Select the combination of letters, that when sequentially placed in the blanks of the given series, will complete the series.
Z _ S M _ Q _ M Z Q _ M _ Q _ M

Let's count the total number of letters and blanks in the series. The series has 16 elements. Let's try dividing it into 4 groups of 4 elements each:
Z_SM∣_Q_M∣ZQ_M∣_Q_M
By comparing the blocks, we can see a pattern where alternative blocks are identical or a single repeating block of 4 letters forms the sequence. Let's check the third block: ZQ_M. Comparing it to the first block Z_SM, we deduce that the pattern could be ZQSM.

Let's substitute ZQSM as the repeating block across all groups:

Z[Q]SM→ZQSM

[Z]Q[S]M→ZQSM

ZQ[S]M→ZQSM

[Z]Q[S]M→ZQSM

The sequential letters placed in the blanks are: Q, Z, S, S, Z, S.
This corresponds to Option D.