Which letter cluster will replace the question mark (?) to complete the given series?

GLYC, FNVG, (?), DRPO, CTMS

Given: GLYC, FNVG, (?), DRPO, CTMS

Logic: The first position follows the pattern of decreasing by 1. The second position follows the pattern of increasing by 2. The third position follows the pattern of decreasing by 3. The fourth position follows the pattern of increasing by 4.

First position: G to F: -1 F to E: -1 E to D: -1 D to C: -1

Second position: L to N: +2 N to P: +2 P to R: +2 R to T: +2

Third position: Y to V: -3 V to S: -3 S to P: -3 P to M: -3

Second position: C to G: +4 G to K: +4 K to O: +4 O to S: +4

Thus, the missing term in the series will be: EPSK. ​​