The first letters are in alphabetical order with a letter skipped in between each segment: C, E, G, I, K. The second and third letters are repeated; they are also in order with a skipped letter: M, O, Q, S, U.
CMM, EOO, GQQ, _____, KUU

  - MSQ
  - IPS
  - IIS
  - ISS
  - POL