Of these answers the only one that is really plausible is A. C is completely untrue. D is untrue for most soldiers most of the time. You could argue that B is true, but it is a stretch.
Of course, you could argue that there were not enough ground forces. If the US had sent 5 million soldiers, it might have been enough to simply blanket Vietnam and end the war. However, this is not a feasible thing to do. As it was, the US put as many as 600,000 troops into Vietnam at a time. It is hard to argue that this is too few.
What really made the war hard was the difficulty of identifying the enemy. It was hard to know who the enemy was and therefore it was hard to destroy them. This is a major reason that is often given for the difficulty of winning the ground war in Vietnam.