D

To solve the question, we need to analyze each condition regarding the net force and its effect on momentum:
(i) If the net force is constant, then the momentum may change its direction while the particle moves. Hence, matched with option [A].
(ii) If the net force is changing in magnitude, the momentum must change its magnitude which aligns with option [B].
(iii) If the net force is changing in direction, then it will change the direction of the momentum, which aligns with option [C].
(iv) If the net force is zero, then it is implied that the momentum must not change its direction. Therefore, the correct match for (iv) is option [D].
Summarized Matches: (i) A, (ii) B, (iii) C, (iv) D.
Hence, the final answer is for (iv) which matches with D.