Select the combination of letters that when sequentially placed in the gaps of the given letter series will complete the series.
M_Q_D M C_Y_M C_Y D

Let's look closely at the letters given in the series: M _ Q _ D M C _ Y _ M C _ Y D
Total number of slots is 15. Let's try dividing them into 3 groups of 5 letters each:
M _ Q _ D | M C _ Y _ | M C _ Y D

Comparing the third group M C _ Y D with the second group M C _ Y _, we can deduce that the last letter of the group is D. So the pattern seems to be M C Q Y D.

Let's test if M C Q Y D works for all groups:

1st group: M C Q Y D

2nd group: M C Q Y D

3rd group: M C Q Y D

The pattern holds perfectly. The missing letters in order are: C, Y, Q, D, Q.