The missing term in the series is MDU.
First Letter
The first letters increase by 2 steps in alphabetical order:
E (+2) → G
G (+2) → I
I (+2) → K
K (+2) → M
Second Letter
The second letters increase by 2 steps in alphabetical order:
Third Letter
The third letters decrease by 2 steps in alphabetical order:
M (+2) → O
O (+2) → Q
Q (+2) → S
S (+2) → U
Combining the final letters gives MDU