Why was the ground war in Vietnam so difficult to fight?A) Civilians often joined the Vietcong and U.S. soldiers often could not identify the enemy...

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.