Answer: Object A
Explanation: 
If object A has half the mass of object B, that is
Ma = 1/2Mb
but object A moves three times as fast as object B. That is 
Va = 3Vb
Momentum is the product of mass and velocity. (MV)
Momentum of A = Ma × Va
 = 1/2Mb × 3Vb
= 3/2MbVb
Momentum of A = 1.5 MbVb
Momentum of B = MbVb 
Therefore, object A has the greater momentum.