Q1F, S2E, U6D, W24C, _______      

Observations:

1. The first letter progresses alphabetically:
   • Q → S → U → W → ?
   • The sequence skips one letter each time (Q, S, U, W).
   • The next letter would be Y.
2. The number:
   • 1 → 2 → 6 → 24 → ?
   • This sequence grows significantly, suggesting multiplication.
   • 1 × 2 = 2, 2 × 3 = 6, 6 × 4 = 24.
   • Following this pattern, 24 × 5 = 120.
3. The last letter moves backward alphabetically:
   • F → E → D → C → ?
   • The sequence is decrementing by 1 each time (F, E, D, C).
   • The next letter would be B.

Conclusion:

The missing segment in the sequence is Y120B.