First let us examine our options. For B, the law of inertia state that an object will keep on moving unless acted upon an external force. And I am thinking this is not the appropriate answer here. Also for C momentum and mass are not the same. Momentum is energy. Momentum cannot be converted to mass. A is the answer since momentum. When the ball hits the box, the box will move and decreases the velocity of the rock. Here momentum is transferred.