Concept for solving this question:
Stage S_{i} is allocated to instruction I_{i }iff
1. Eligibility: I_{i} must have completed S_{i-1} ... X time
2. Availability: I_{i-1 }must have released S_{i }... Y time
Time of allocation = Max(X,Y)
In first iteration:
I1 completes after 5th cycle
I2 completes after 10th cycle
I3 completes after 13th cycle
I4 completes after 15th cycle
In second iteration
I1 completes after 16th cycle
I2 completes after 18th cycle
I3 completes after 21th cycle
I4 completes after 23th cycle
Answer : B