The formula for momentum is p=mv where p is the momentum (kgms-1), m is the mass (kg) and v is the velocity (ms-1). So, to work out the momentum, we just multiply these numbers together and work out which is the largest.
1) 12kgms-1
2) 10kgms-1
3) 27kgms-1
4) 16kgms-1
Therefore the object with the greatest momentum is 3 - a 9kg mass moving at 3m/s