B

Step 1: If the net force is constant, the momentum of the object may change its direction without changing its magnitude since momentum is the product of mass and velocity, and a change in velocity can occur through a change in direction while keeping the speed constant.
Step 2: If the net force is changing in magnitude, the momentum must change its magnitude as a result of the net force acting differently.
Step 3: If the net force is changing in direction, the momentum may not change its direction without an angular or lateral acceleration acting.
Step 4: If the net force is zero, then the momentum must not change its direction since the object is in a state of equilibrium.
Therefore, the correct pairing is:
(i) - [A],
(ii) - [B],
(iii) - [C],
(iv) - [D].