Momentum of a ball of mass 2 kg moving at 30 m/s = 2 kg × 30 m/s = 60 kg m/s
Momentum of a ball of mass 3 kg moving at 20 m/s = 3 kg × 20 m/s = 60 kg m/s
Momentum of a ball of mass 4 kg moving at 30 m/s = 4 kg × 30 m/s = 120 kg m/s
Momentum of a ball of mass 5 kg moving at 20 m/s = 5 kg × 20 m/s = 100 kg m/s
So, highest momentum is of a ball of mass 4 kg moving at 30 m/s (120 kg m/s).